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>

lib/hooks/useKeyboardShortcuts.ts

@@ -0,0 +1,65 @@
+import { useEffect } from 'react';
+
+export interface KeyboardShortcut {
+  key: string;
+  ctrl?: boolean;
+  alt?: boolean;
+  shift?: boolean;
+  description: string;
+  action: () => void;
+}
+
+/**
+ * Hook for managing keyboard shortcuts
+ */
+export function useKeyboardShortcuts(shortcuts: KeyboardShortcut[], enabled: boolean = true) {
+  useEffect(() => {
+    if (!enabled) return;
+
+    const handleKeyDown = (event: KeyboardEvent) => {
+      // Find matching shortcut
+      const shortcut = shortcuts.find((s) => {
+        const keyMatch = s.key.toLowerCase() === event.key.toLowerCase();
+        const ctrlMatch = s.ctrl ? (event.ctrlKey || event.metaKey) : !event.ctrlKey && !event.metaKey;
+        const altMatch = s.alt ? event.altKey : !event.altKey;
+        const shiftMatch = s.shift ? event.shiftKey : !event.shiftKey;
+
+        return keyMatch && ctrlMatch && altMatch && shiftMatch;
+      });
+
+      if (shortcut) {
+        event.preventDefault();
+        shortcut.action();
+      }
+    };
+
+    window.addEventListener('keydown', handleKeyDown);
+    return () => window.removeEventListener('keydown', handleKeyDown);
+  }, [shortcuts, enabled]);
+}
+
+/**
+ * Format shortcut key combination for display
+ */
+export function formatShortcut(shortcut: KeyboardShortcut): string {
+  const parts: string[] = [];
+
+  // Use Cmd on Mac, Ctrl on others
+  const isMac = typeof window !== 'undefined' && /Mac|iPhone|iPod|iPad/.test(navigator.platform);
+
+  if (shortcut.ctrl) {
+    parts.push(isMac ? '⌘' : 'Ctrl');
+  }
+
+  if (shortcut.alt) {
+    parts.push(isMac ? '⌥' : 'Alt');
+  }
+
+  if (shortcut.shift) {
+    parts.push(isMac ? '⇧' : 'Shift');
+  }
+
+  parts.push(shortcut.key.toUpperCase());
+
+  return parts.join(' + ');
+}