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/file-menu.tsx

@@ -0,0 +1,135 @@
+'use client';
+
+import { useState, useRef } from 'react';
+import { useFileOperations } from '@/hooks/use-file-operations';
+import { ExportDialog } from '@/components/modals/export-dialog';
+import { NewImageDialog } from '@/components/modals/new-image-dialog';
+import {
+  FileImage,
+  FolderOpen,
+  Download,
+  Save,
+  ChevronDown,
+} from 'lucide-react';
+import { cn } from '@/lib/utils';
+
+export function FileMenu() {
+  const {
+    createNewImage,
+    handleFileInput,
+    exportImage,
+    saveProject,
+  } = useFileOperations();
+
+  const [isMenuOpen, setIsMenuOpen] = useState(false);
+  const [isNewDialogOpen, setIsNewDialogOpen] = useState(false);
+  const [isExportDialogOpen, setIsExportDialogOpen] = useState(false);
+  const fileInputRef = useRef<HTMLInputElement>(null);
+
+  const handleOpenFile = () => {
+    fileInputRef.current?.click();
+    setIsMenuOpen(false);
+  };
+
+  const handleFileChange = async (e: React.ChangeEvent<HTMLInputElement>) => {
+    const file = e.target.files?.[0];
+    if (file) {
+      await handleFileInput(file);
+    }
+    // Reset input
+    if (fileInputRef.current) {
+      fileInputRef.current.value = '';
+    }
+  };
+
+  const handleSaveProject = () => {
+    saveProject('project');
+    setIsMenuOpen(false);
+  };
+
+  return (
+    <>
+      <div className="relative">
+        <button
+          onClick={() => setIsMenuOpen(!isMenuOpen)}
+          className="flex items-center gap-2 px-3 py-2 hover:bg-accent rounded-md transition-colors"
+        >
+          <span className="text-sm font-medium">File</span>
+          <ChevronDown className="h-4 w-4" />
+        </button>
+
+        {isMenuOpen && (
+          <>
+            <div
+              className="fixed inset-0 z-10"
+              onClick={() => setIsMenuOpen(false)}
+            />
+            <div className="absolute left-0 top-full mt-1 w-48 bg-card border border-border rounded-md shadow-lg z-20">
+              <button
+                onClick={() => {
+                  setIsNewDialogOpen(true);
+                  setIsMenuOpen(false);
+                }}
+                className="flex items-center gap-3 w-full px-4 py-2 text-sm hover:bg-accent transition-colors"
+              >
+                <FileImage className="h-4 w-4" />
+                New Image
+              </button>
+
+              <button
+                onClick={handleOpenFile}
+                className="flex items-center gap-3 w-full px-4 py-2 text-sm hover:bg-accent transition-colors"
+              >
+                <FolderOpen className="h-4 w-4" />
+                Open...
+              </button>
+
+              <div className="h-px bg-border my-1" />
+
+              <button
+                onClick={handleSaveProject}
+                className="flex items-center gap-3 w-full px-4 py-2 text-sm hover:bg-accent transition-colors"
+              >
+                <Save className="h-4 w-4" />
+                Save Project
+              </button>
+
+              <button
+                onClick={() => {
+                  setIsExportDialogOpen(true);
+                  setIsMenuOpen(false);
+                }}
+                className="flex items-center gap-3 w-full px-4 py-2 text-sm hover:bg-accent transition-colors"
+              >
+                <Download className="h-4 w-4" />
+                Export Image...
+              </button>
+            </div>
+          </>
+        )}
+      </div>
+
+      {/* Hidden file input */}
+      <input
+        ref={fileInputRef}
+        type="file"
+        accept="image/*,.paint"
+        onChange={handleFileChange}
+        className="hidden"
+      />
+
+      {/* Dialogs */}
+      <NewImageDialog
+        isOpen={isNewDialogOpen}
+        onClose={() => setIsNewDialogOpen(false)}
+        onCreate={createNewImage}
+      />
+
+      <ExportDialog
+        isOpen={isExportDialogOpen}
+        onClose={() => setIsExportDialogOpen(false)}
+        onExport={exportImage}
+      />
+    </>
+  );
+}