feat: add comprehensive Filters & Adjustments dialog with live preview

Implements professional image filter system with real-time preview:

**Adjustments Dialog** (`components/modals/adjustments-dialog.tsx`):
- Live preview canvas (max 400x400px) with real-time filter updates
- Organized filter categories: Color Adjustments, Filters, Effects
- 10 filter types with adjustable parameters:
  - **Brightness**: -100 to +100 adjustment
  - **Contrast**: -100 to +100 adjustment
  - **Hue/Saturation**: Hue rotation (±180°), Saturation (±100%), Lightness (±100%)
  - **Blur**: Gaussian blur with radius 1-50px
  - **Sharpen**: Amount 0-100%
  - **Invert**: One-click color inversion
  - **Grayscale**: Convert to monochrome
  - **Sepia**: Vintage sepia tone effect
  - **Threshold**: Binary black/white with adjustable threshold
  - **Posterize**: Reduce colors to 2-256 levels
- Slider controls for all adjustable parameters
- Reset button to restore default values
- Apply/Cancel actions with undo support
- Uses FilterCommand for history integration

**Image Menu** (`components/editor/image-menu.tsx`):
- New "Image" menu in header bar next to "File"
- "Filters & Adjustments..." menu item opens dialog
- Organized location for image-related operations
- Extensible for future image operations

**Integration**:
- Added Image menu to editor layout header
- Positioned between title and controls
- Keyboard-accessible with proper ARIA labels

**Technical Features**:
- Async filter application using Web Workers for large images
- Non-destructive editing with undo/redo support
- Real-time preview updates as sliders adjust
- FilterCommand integration for history
- Canvas cloning for before/after states
- Optimized preview rendering (scaled to 400px max)

**User Experience**:
- Modal dialog with backdrop
- Sidebar filter list with hover states
- Large preview area showing filter results
- Smooth slider interactions
- Instant visual feedback
- Professional Photoshop-like interface

All filters tested and working with undo/redo support.

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

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

components/editor/editor-layout.tsx

@@ -5,6 +5,7 @@ import { useCanvasStore, useLayerStore } from '@/store';
 import { useHistoryStore } from '@/store/history-store';
 import { CanvasWithTools } from '@/components/canvas/canvas-with-tools';
 import { FileMenu } from './file-menu';
+import { ImageMenu } from './image-menu';
 import { ToolOptions } from './tool-options';
 import { PanelDock } from './panel-dock';
 import { ThemeToggle } from './theme-toggle';
@@ -101,12 +102,13 @@ export function EditorLayout() {
 
       {/* Header Bar */}
       <div className="flex h-12 items-center justify-between border-b border-border bg-card px-4">
-        {/* Left: Title and File Menu */}
-        <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">
+        {/* Left: Title and Menus */}
+        <div className="flex items-center gap-2">
+          <h1 className="text-lg font-semibold bg-gradient-to-r from-primary via-accent to-primary bg-clip-text text-transparent mr-2">
             Paint UI
           </h1>
           <FileMenu />
+          <ImageMenu />
         </div>
 
         {/* Right: Controls */}

components/editor/image-menu.tsx

@@ -0,0 +1,63 @@
+'use client';
+
+import { useState } from 'react';
+import { AdjustmentsDialog } from '@/components/modals/adjustments-dialog';
+import {
+  Sliders,
+  ChevronDown,
+} from 'lucide-react';
+
+export function ImageMenu() {
+  const [isMenuOpen, setIsMenuOpen] = useState(false);
+  const [isAdjustmentsOpen, setIsAdjustmentsOpen] = useState(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 focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2"
+          aria-label="Image menu"
+          aria-expanded={isMenuOpen}
+          aria-haspopup="menu"
+        >
+          <span className="text-sm font-medium">Image</span>
+          <ChevronDown className="h-4 w-4" aria-hidden="true" />
+        </button>
+
+        {isMenuOpen && (
+          <>
+            <div
+              className="fixed inset-0 z-10"
+              onClick={() => setIsMenuOpen(false)}
+              aria-hidden="true"
+            />
+            <div
+              className="absolute left-0 top-full mt-1 w-56 bg-card border border-border rounded-md shadow-lg z-20"
+              role="menu"
+              aria-label="Image operations"
+            >
+              <button
+                onClick={() => {
+                  setIsAdjustmentsOpen(true);
+                  setIsMenuOpen(false);
+                }}
+                className="flex items-center gap-3 w-full px-4 py-2 text-sm hover:bg-accent transition-colors focus:outline-none focus:bg-accent text-left"
+                role="menuitem"
+              >
+                <Sliders className="h-4 w-4" aria-hidden="true" />
+                Filters & Adjustments...
+              </button>
+            </div>
+          </>
+        )}
+      </div>
+
+      {/* Adjustments Dialog */}
+      <AdjustmentsDialog
+        isOpen={isAdjustmentsOpen}
+        onClose={() => setIsAdjustmentsOpen(false)}
+      />
+    </>
+  );
+}