feat: implement UI state persistence and theme toggle

Major improvements to UI state management and user preferences:

- Add theme toggle with dark/light mode support
- Implement Zustand persist middleware for UI state
- Add ui-store for panel layout preferences (dock width, heights, tabs)
- Persist tool settings (active tool, size, opacity, hardness, etc.)
- Persist canvas view preferences (grid, rulers, snap-to-grid)
- Persist shape tool settings
- Persist collapsible section states
- Fix canvas coordinate transformation for centered rendering
- Constrain checkerboard and grid to canvas bounds
- Add icons to all tab buttons and collapsible sections
- Restructure panel-dock to use persisted state

Storage impact: ~3.5KB total across all preferences
Storage keys: tool-storage, canvas-view-storage, shape-storage, ui-storage

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

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

components/editor/editor-layout.tsx

@@ -7,6 +7,7 @@ import { CanvasWithTools } from '@/components/canvas/canvas-with-tools';
 import { FileMenu } from './file-menu';
 import { ToolOptions } from './tool-options';
 import { PanelDock } from './panel-dock';
+import { ThemeToggle } from './theme-toggle';
 import { ToolPalette } from '@/components/tools';
 import { useKeyboardShortcuts } from '@/hooks/use-keyboard-shortcuts';
 import { useFileOperations } from '@/hooks/use-file-operations';
@@ -155,6 +156,11 @@ export function EditorLayout() {
 
           <div className="w-px h-6 bg-border mx-1" />
 
+          {/* Theme Toggle */}
+          <ThemeToggle />
+
+          <div className="w-px h-6 bg-border mx-1" />
+
           {/* New Layer */}
           <button
             onClick={handleNewLayer}
@@ -167,7 +173,7 @@ export function EditorLayout() {
       </div>
 
       {/* Tool Adjustments Bar */}
-      <div className="flex h-10 items-center border-b border-border bg-card/50 px-4">
+      <div className="flex h-10 items-center border-b border-border bg-card/50">
         <ToolOptions />
       </div>