feat: add comprehensive keyboard shortcuts system

Keyboard shortcuts:
- **Ctrl/Cmd + O**: Open file dialog
- **Ctrl/Cmd + Enter**: Start conversion
- **Ctrl/Cmd + S**: Download results (ZIP if multiple)
- **Ctrl/Cmd + R**: Reset converter
- **Ctrl/Cmd + /**: Show keyboard shortcuts help
- **?**: Show keyboard shortcuts help
- **Escape**: Close shortcuts modal

Implementation:
- Created useKeyboardShortcuts custom hook
- Platform-aware keyboard handling (Cmd on Mac, Ctrl elsewhere)
- KeyboardShortcutsModal component with beautiful UI
- Floating keyboard icon button (bottom-right)
- Visual shortcut display with kbd tags
- Context-aware shortcut execution (respects disabled states)
- Prevents default browser behavior for shortcuts

User experience:
- Floating help button always accessible
- Clean modal with shortcut descriptions
- Platform-specific key symbols (⌘ on Mac)
- Shortcuts disabled when modal is open
- Clear visual feedback for shortcuts
- Non-intrusive button placement

Features:
- Smart ref forwarding to FileUpload component
- Conditional shortcut execution based on app state
- Professional kbd styling for key combinations
- Responsive modal with backdrop
- Smooth animations

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

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

components/ui/KeyboardShortcutsModal.tsx

@@ -0,0 +1,66 @@
+'use client';
+
+import * as React from 'react';
+import { X, Keyboard } from 'lucide-react';
+import { Button } from './Button';
+import { Card } from './Card';
+import { formatShortcut, type KeyboardShortcut } from '@/lib/hooks/useKeyboardShortcuts';
+
+interface KeyboardShortcutsModalProps {
+  shortcuts: KeyboardShortcut[];
+  isOpen: boolean;
+  onClose: () => void;
+}
+
+export function KeyboardShortcutsModal({ shortcuts, isOpen, onClose }: KeyboardShortcutsModalProps) {
+  if (!isOpen) return null;
+
+  return (
+    <>
+      {/* Backdrop */}
+      <div
+        className="fixed inset-0 bg-black/50 z-40 animate-fadeIn"
+        onClick={onClose}
+      />
+
+      {/* Modal */}
+      <div className="fixed inset-0 z-50 flex items-center justify-center p-4">
+        <Card className="w-full max-w-md max-h-[80vh] overflow-y-auto animate-fadeIn">
+          <div className="p-6">
+            {/* Header */}
+            <div className="flex items-center justify-between mb-6">
+              <div className="flex items-center gap-2">
+                <Keyboard className="h-5 w-5 text-primary" />
+                <h2 className="text-lg font-semibold">Keyboard Shortcuts</h2>
+              </div>
+              <Button variant="ghost" size="icon" onClick={onClose}>
+                <X className="h-4 w-4" />
+              </Button>
+            </div>
+
+            {/* Shortcuts List */}
+            <div className="space-y-3">
+              {shortcuts.map((shortcut, index) => (
+                <div
+                  key={index}
+                  className="flex items-center justify-between py-2 border-b border-border last:border-0"
+                >
+                  <span className="text-sm text-foreground">{shortcut.description}</span>
+                  <kbd className="px-2 py-1 text-xs font-semibold text-foreground bg-muted border border-border rounded">
+                    {formatShortcut(shortcut)}
+                  </kbd>
+                </div>
+              ))}
+            </div>
+
+            {/* Footer */}
+            <div className="mt-6 text-xs text-muted-foreground text-center">
+              Press <kbd className="px-1.5 py-0.5 text-xs font-semibold bg-muted border border-border rounded">Esc</kbd> or{' '}
+              <kbd className="px-1.5 py-0.5 text-xs font-semibold bg-muted border border-border rounded">?</kbd> to close
+            </div>
+          </div>
+        </Card>
+      </div>
+    </>
+  );
+}