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/shapes/shape-panel.tsx

@@ -63,13 +63,7 @@ export function ShapePanel() {
   };
 
   return (
-    <div className="w-64 border-l border-border bg-card flex flex-col">
-      {/* Header */}
-      <div className="flex items-center gap-2 border-b border-border p-3">
-        <Square className="h-5 w-5 text-primary" />
-        <h2 className="text-sm font-semibold">Shapes</h2>
-      </div>
-
+    <div className="w-full border-l border-border bg-card flex flex-col">
       {/* Shape Types */}
       <div className="border-b border-border p-3">
         <h3 className="text-xs font-semibold text-muted-foreground uppercase mb-2">