feat: ensure all category components use consistent hex colors

Update all category-related UI components to use getCategoryColorHex() function
instead of CSS variables for consistent color application across the app.

Changes:
- MainConverter: category buttons now use hex colors for background/border
- MainConverter: quick result display uses hex color for text and border
- MainConverter: result cards use hex color for left border
- CommandPalette: measure commands use hex colors for color indicators
- SearchUnits: category color dots use hex colors

This ensures all category colors are consistently applied using the same
hex color values defined in the OKLCH color palette.

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

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

components/ui/CommandPalette.tsx

@@ -7,6 +7,7 @@ import {
   getAllMeasures,
   formatMeasureName,
   getCategoryColor,
+  getCategoryColorHex,
   type Measure,
 } from '@/lib/units';
 import { getHistory, getFavorites } from '@/lib/storage';
@@ -60,7 +61,7 @@ export default function CommandPalette({
     icon: Hash,
     action: () => onSelectMeasure(measure),
     keywords: ['convert', measure, formatMeasureName(measure).toLowerCase()],
-    color: getCategoryColor(measure),
+    color: getCategoryColorHex(measure),
   }));
 
   const allCommands = [...commands, ...measureCommands];
@@ -187,7 +188,7 @@ export default function CommandPalette({
                       <div
                         className="w-5 h-5 rounded flex-shrink-0"
                         style={{
-                          backgroundColor: `var(--color-${command.color})`,
+                          backgroundColor: command.color,
                         }}
                       />
                     ) : (