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/canvas/canvas-wrapper.tsx

@@ -51,8 +51,8 @@ export function CanvasWrapper() {
     ctx.scale(zoom, zoom);
     ctx.translate(-width / 2, -height / 2);
 
-    // Draw checkerboard background
-    drawCheckerboard(ctx, 10, '#ffffff', '#e0e0e0');
+    // Draw checkerboard background (only within canvas bounds)
+    drawCheckerboard(ctx, 10, '#ffffff', '#e0e0e0', width, height);
 
     // Draw background color if not transparent
     if (backgroundColor && backgroundColor !== 'transparent') {
@@ -76,9 +76,9 @@ export function CanvasWrapper() {
     ctx.globalAlpha = 1;
     ctx.globalCompositeOperation = 'source-over';
 
-    // Draw grid if enabled
+    // Draw grid if enabled (only within canvas bounds)
     if (showGrid) {
-      drawGrid(ctx, gridSize, 'rgba(0, 0, 0, 0.15)');
+      drawGrid(ctx, gridSize, 'rgba(0, 0, 0, 0.15)', width, height);
     }
 
     // Draw selection if active