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>

components/editor/editor-layout.tsx

@@ -6,21 +6,32 @@ import { useHistoryStore } from '@/store/history-store';
 import { CanvasWithTools } from '@/components/canvas/canvas-with-tools';
 import { LayersPanel } from '@/components/layers/layers-panel';
 import { HistoryPanel } from './history-panel';
+import { FileMenu } from './file-menu';
 import { ToolPalette, ToolSettings } from '@/components/tools';
 import { ColorPanel } from '@/components/colors';
 import { useKeyboardShortcuts } from '@/hooks/use-keyboard-shortcuts';
+import { useFileOperations } from '@/hooks/use-file-operations';
+import { useDragDrop } from '@/hooks/use-drag-drop';
+import { useClipboard } from '@/hooks/use-clipboard';
 import { createLayerWithHistory } from '@/lib/layer-operations';
-import { Plus, ZoomIn, ZoomOut, Maximize, Undo, Redo } from 'lucide-react';
+import { Plus, ZoomIn, ZoomOut, Maximize, Undo, Redo, Upload } from 'lucide-react';
 import { cn } from '@/lib/utils';
 
 export function EditorLayout() {
   const { zoom, zoomIn, zoomOut, zoomToFit } = useCanvasStore();
   const { layers } = useLayerStore();
   const { undo, redo, canUndo, canRedo } = useHistoryStore();
+  const { handleDataTransfer } = useFileOperations();
 
   // Enable keyboard shortcuts
   useKeyboardShortcuts();
 
+  // Enable drag & drop
+  const { isDragging, handleDragOver, handleDragLeave, handleDrop } = useDragDrop(handleDataTransfer);
+
+  // Enable clipboard paste
+  useClipboard(handleDataTransfer);
+
   // Initialize with a default layer (without history)
   useEffect(() => {
     if (layers.length === 0) {
@@ -50,13 +61,34 @@ export function EditorLayout() {
   };
 
   return (
-    <div className="flex h-screen flex-col overflow-hidden">
+    <div
+      className="flex h-screen flex-col overflow-hidden"
+      onDragOver={handleDragOver}
+      onDragLeave={handleDragLeave}
+      onDrop={handleDrop}
+    >
+      {/* Drag overlay */}
+      {isDragging && (
+        <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/50 backdrop-blur-sm">
+          <div className="flex flex-col items-center gap-4 p-8 bg-card border-2 border-dashed border-primary rounded-lg">
+            <Upload className="h-16 w-16 text-primary" />
+            <p className="text-lg font-semibold text-foreground">
+              Drop image or project file
+            </p>
+            <p className="text-sm text-muted-foreground">
+              Supports: PNG, JPG, WEBP, .paint
+            </p>
+          </div>
+        </div>
+      )}
+
       {/* Toolbar */}
       <div className="flex h-14 items-center justify-between border-b border-border bg-card px-4">
-        <div className="flex items-center gap-2">
+        <div className="flex items-center gap-4">
           <h1 className="text-lg font-semibold bg-gradient-to-r from-primary via-accent to-primary bg-clip-text text-transparent">
             Paint UI
           </h1>
+          <FileMenu />
         </div>
 
         {/* History controls */}