feat: implement Phase 4 - Command palette, visual comparison, and polish

Add final layer of advanced features for spectacular UX:

⌨️ Command Palette (CommandPalette.tsx):
- Trigger with Ctrl/Cmd+K keyboard shortcut
- Search and execute commands instantly
- Quick access to all 23 measurement categories
- Theme switching commands (light/dark/system)
- Keyboard navigation (↑/↓ arrows, Enter to select)
- Escape to close
- Color-coded category commands
- Backdrop blur overlay
- Animated scale-in entrance
- Footer with keyboard hints
- Smart filtering by keywords

📊 Visual Comparison (VisualComparison.tsx):
- Toggle between grid and chart view
- Horizontal bar chart showing magnitude differences
- Animated bars with 500ms transitions
- Auto-calculated percentages relative to max value
- Color-coded bars matching category colors
- Tabular number formatting
- Clean, minimal design

🔄 Unit Swap Functionality:
- Swap button between From/To units
- ArrowLeftRight icon
- Automatically converts value on swap
- Two-unit quick converter mode
- Large result display with color accent
- Responsive layout

📱 Footer Component (Footer.tsx):
- Three-column grid layout (About, Features, Links)
- GitHub repository link
- convert-units library attribution
- Keyboard shortcuts reference
- Feature highlights
- Made with ❤️ message
- Responsive design (stacks on mobile)
- Copyright notice with current year
- Smooth hover transitions

🎨 Enhanced MainConverter:
- From/To unit selector with swap button
- Quick result display between selectors
- Toggle between grid and chart views
- BarChart3 icon for view switcher
- Integrated command palette
- Better spacing and layout
- Target unit state management
- Auto-update target unit on measure change

✨ Enhanced Page Layout:
- Sticky header with backdrop blur
- Flexbox layout for footer at bottom
- Keyboard shortcuts hint (/ and Ctrl+K)
- Improved header spacing
- Better visual hierarchy

Features Now Complete:
✅ Command palette with Ctrl+K
✅ Visual comparison bar charts
✅ Unit swap functionality
✅ Professional footer
✅ From/To quick converter
✅ Chart/Grid view toggle
✅ Sticky navigation header
✅ Full keyboard navigation

The app is now feature-complete with:
- 23 measurement categories
- 187 individual units
- Real-time conversion
- Fuzzy search (/)
- Command palette (Ctrl+K)
- Dark mode
- Conversion history
- Favorites & copy
- Visual comparisons
- Unit swapping
- Complete footer

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/converter/MainConverter.tsx

@@ -1,12 +1,14 @@
 'use client';
 
 import { useState, useEffect, useCallback } from 'react';
-import { Copy, Star, Check } from 'lucide-react';
+import { Copy, Star, Check, ArrowLeftRight, BarChart3 } from 'lucide-react';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
 import { Input } from '@/components/ui/input';
 import { Button } from '@/components/ui/button';
 import SearchUnits from './SearchUnits';
 import ConversionHistory from './ConversionHistory';
+import VisualComparison from './VisualComparison';
+import CommandPalette from '@/components/ui/CommandPalette';
 import {
   getAllMeasures,
   getUnitsForMeasure,
@@ -23,10 +25,12 @@ import { saveToHistory, getFavorites, toggleFavorite } from '@/lib/storage';
 export default function MainConverter() {
   const [selectedMeasure, setSelectedMeasure] = useState<Measure>('length');
   const [selectedUnit, setSelectedUnit] = useState<string>('m');
+  const [targetUnit, setTargetUnit] = useState<string>('ft');
   const [inputValue, setInputValue] = useState<string>('1');
   const [conversions, setConversions] = useState<ConversionResult[]>([]);
   const [favorites, setFavorites] = useState<string[]>([]);
   const [copiedUnit, setCopiedUnit] = useState<string | null>(null);
+  const [showVisualComparison, setShowVisualComparison] = useState(false);
 
   const measures = getAllMeasures();
   const units = getUnitsForMeasure(selectedMeasure);
@@ -52,9 +56,24 @@ export default function MainConverter() {
     const availableUnits = getUnitsForMeasure(selectedMeasure);
     if (availableUnits.length > 0) {
       setSelectedUnit(availableUnits[0]);
+      setTargetUnit(availableUnits[1] || availableUnits[0]);
     }
   }, [selectedMeasure]);
 
+  // Swap units
+  const handleSwapUnits = useCallback(() => {
+    const temp = selectedUnit;
+    setSelectedUnit(targetUnit);
+    setTargetUnit(temp);
+
+    // Convert the value
+    const numValue = parseNumberInput(inputValue);
+    if (numValue !== null) {
+      const converted = convertUnit(numValue, selectedUnit, targetUnit);
+      setInputValue(converted.toString());
+    }
+  }, [selectedUnit, targetUnit, inputValue]);
+
   // Copy to clipboard
   const copyToClipboard = useCallback(async (value: number, unit: string) => {
     try {
@@ -105,6 +124,12 @@ export default function MainConverter() {
 
   return (
     <div className="w-full max-w-6xl mx-auto space-y-6">
+      {/* Command Palette */}
+      <CommandPalette
+        onSelectMeasure={setSelectedMeasure}
+        onSelectUnit={handleSearchSelect}
+      />
+
       {/* Search */}
       <div className="flex justify-center">
         <SearchUnits onSelectUnit={handleSearchSelect} />
@@ -144,7 +169,7 @@ export default function MainConverter() {
           <CardTitle>Convert {formatMeasureName(selectedMeasure)}</CardTitle>
         </CardHeader>
         <CardContent className="space-y-4">
-          <div className="flex gap-4">
+          <div className="flex gap-2 items-end">
             <div className="flex-1">
               <label className="text-sm font-medium mb-2 block">Value</label>
               <Input
@@ -156,8 +181,8 @@ export default function MainConverter() {
                 className="text-lg"
               />
             </div>
-            <div className="w-48">
-              <label className="text-sm font-medium mb-2 block">From Unit</label>
+            <div className="w-40">
+              <label className="text-sm font-medium mb-2 block">From</label>
               <select
                 value={selectedUnit}
                 onChange={(e) => setSelectedUnit(e.target.value)}
@@ -170,18 +195,69 @@ export default function MainConverter() {
                 ))}
               </select>
             </div>
+            <Button
+              variant="outline"
+              size="icon"
+              onClick={handleSwapUnits}
+              className="flex-shrink-0"
+              title="Swap units"
+            >
+              <ArrowLeftRight className="h-4 w-4" />
+            </Button>
+            <div className="w-40">
+              <label className="text-sm font-medium mb-2 block">To</label>
+              <select
+                value={targetUnit}
+                onChange={(e) => setTargetUnit(e.target.value)}
+                className="flex h-10 w-full rounded-md border border-input bg-background px-3 py-2 text-sm ring-offset-background focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-2"
+              >
+                {units.map((unit) => (
+                  <option key={unit} value={unit}>
+                    {unit}
+                  </option>
+                ))}
+              </select>
+            </div>
           </div>
+
+          {/* Quick result */}
+          {parseNumberInput(inputValue) !== null && (
+            <div className="p-4 rounded-lg bg-accent/50 border-l-4" style={{
+              borderLeftColor: `var(--color-${getCategoryColor(selectedMeasure)})`,
+            }}>
+              <div className="text-sm text-muted-foreground">Result</div>
+              <div className="text-3xl font-bold mt-1">
+                {formatNumber(convertUnit(parseNumberInput(inputValue)!, selectedUnit, targetUnit))} {targetUnit}
+              </div>
+            </div>
+          )}
         </CardContent>
       </Card>
 
       {/* Results */}
       <Card>
         <CardHeader>
-          <CardTitle>Conversions</CardTitle>
+          <div className="flex items-center justify-between">
+            <CardTitle>All Conversions</CardTitle>
+            <Button
+              variant="outline"
+              size="sm"
+              onClick={() => setShowVisualComparison(!showVisualComparison)}
+            >
+              <BarChart3 className="h-4 w-4 mr-2" />
+              {showVisualComparison ? 'Grid View' : 'Chart View'}
+            </Button>
+          </div>
         </CardHeader>
         <CardContent>
-          <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-4">
-            {conversions.map((conversion) => {
+          {showVisualComparison ? (
+            <VisualComparison
+              conversions={conversions}
+              color={getCategoryColor(selectedMeasure)}
+            />
+          ) : (
+            <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-4">
+              {conversions.map((conversion) => {
               const isFavorite = favorites.includes(conversion.unit);
               const isCopied = copiedUnit === conversion.unit;
 
@@ -235,7 +311,8 @@ export default function MainConverter() {
                 </div>
               );
             })}
-          </div>
+            </div>
+          )}
         </CardContent>
       </Card>
 

components/converter/VisualComparison.tsx

@@ -0,0 +1,60 @@
+'use client';
+
+import { useMemo } from 'react';
+import { type ConversionResult } from '@/lib/units';
+import { formatNumber, cn } from '@/lib/utils';
+
+interface VisualComparisonProps {
+  conversions: ConversionResult[];
+  color: string;
+}
+
+export default function VisualComparison({
+  conversions,
+  color,
+}: VisualComparisonProps) {
+  // Calculate percentages for visual bars
+  const withPercentages = useMemo(() => {
+    if (conversions.length === 0) return [];
+
+    const maxValue = Math.max(...conversions.map(c => Math.abs(c.value)));
+    if (maxValue === 0) return conversions.map(c => ({ ...c, percentage: 0 }));
+
+    return conversions.map(c => ({
+      ...c,
+      percentage: (Math.abs(c.value) / maxValue) * 100,
+    }));
+  }, [conversions]);
+
+  if (conversions.length === 0) {
+    return (
+      <div className="text-center py-8 text-muted-foreground">
+        Enter a value to see conversions
+      </div>
+    );
+  }
+
+  return (
+    <div className="space-y-3">
+      {withPercentages.map(item => (
+        <div key={item.unit} className="space-y-1">
+          <div className="flex items-center justify-between text-sm">
+            <span className="font-medium">{item.unitInfo.plural}</span>
+            <span className="tabular-nums">
+              {formatNumber(item.value)} {item.unit}
+            </span>
+          </div>
+          <div className="h-2 bg-muted rounded-full overflow-hidden">
+            <div
+              className="h-full rounded-full transition-all duration-500 ease-out"
+              style={{
+                width: `${item.percentage}%`,
+                backgroundColor: `var(--color-${color})`,
+              }}
+            />
+          </div>
+        </div>
+      ))}
+    </div>
+  );
+}