feat(phase-6): implement comprehensive file operations system

This commit completes Phase 6 of the paint-ui implementation, adding full
file import/export capabilities with drag-drop and clipboard support.

**New Files:**
- lib/file-utils.ts: Core file operations (open, save, export, project format)
- hooks/use-file-operations.ts: React hook for file operations
- hooks/use-drag-drop.ts: Drag & drop state management
- hooks/use-clipboard.ts: Clipboard paste event handling
- components/editor/file-menu.tsx: File menu dropdown component
- components/modals/export-dialog.tsx: Export dialog with format/quality options
- components/modals/new-image-dialog.tsx: New image dialog with presets
- components/modals/index.ts: Modals barrel export

**Updated Files:**
- components/editor/editor-layout.tsx: Integrated FileMenu, drag-drop overlay, clipboard paste
- components/editor/index.ts: Added file-menu export

**Features:**
- ✨ Create new images with dimension presets (Full HD, HD, 800x600, custom)
- ✨ Open image files (PNG, JPG, WEBP) as new layers
- ✨ Save/load .paint project files (JSON with base64 layer data)
- ✨ Export as PNG/JPEG/WEBP with quality control
- ✨ Drag & drop file upload with visual overlay
- ✨ Clipboard paste support (Ctrl+V)
- ✨ File type validation and error handling
- ✨ DataTransfer API integration for unified file handling

**Project File Format (.paint):**
- JSON structure with version, dimensions, layer metadata
- Base64-encoded PNG data for each layer
- Preserves layer properties (opacity, blend mode, order, visibility)

Build verified: ✓ Compiled successfully in 1233ms

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

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

hooks/use-drag-drop.ts

@@ -0,0 +1,35 @@
+import { useCallback, useState } from 'react';
+
+export function useDragDrop(onDrop: (dataTransfer: DataTransfer) => void) {
+  const [isDragging, setIsDragging] = useState(false);
+
+  const handleDragOver = useCallback((e: React.DragEvent) => {
+    e.preventDefault();
+    e.stopPropagation();
+    setIsDragging(true);
+  }, []);
+
+  const handleDragLeave = useCallback((e: React.DragEvent) => {
+    e.preventDefault();
+    e.stopPropagation();
+    setIsDragging(false);
+  }, []);
+
+  const handleDrop = useCallback(
+    (e: React.DragEvent) => {
+      e.preventDefault();
+      e.stopPropagation();
+      setIsDragging(false);
+
+      onDrop(e.dataTransfer);
+    },
+    [onDrop]
+  );
+
+  return {
+    isDragging,
+    handleDragOver,
+    handleDragLeave,
+    handleDrop,
+  };
+}