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/history-panel.tsx

@@ -8,13 +8,6 @@ export function HistoryPanel() {
 
   return (
     <div className="flex h-full flex-col bg-card">
-      <div className="border-b border-border p-3">
-        <h2 className="flex items-center gap-2 text-sm font-semibold text-card-foreground">
-          <History className="h-4 w-4" />
-          History
-        </h2>
-      </div>
-
       <div className="flex-1 overflow-y-auto p-2 space-y-2">
         {undoStack.length === 0 && redoStack.length === 0 ? (
           <div className="flex h-full items-center justify-center">