feat: add Turing Complete-style puzzle system

Add progressive puzzle mode alongside the existing sandbox:
- 8 levels from basic gates to 2-bit adder
- Truth table verification with pass/fail feedback
- Gate restrictions per level
- Custom components system (save circuits as reusable chips)
- Save/load circuits as JSON
- Level selection sidebar with difficulty ratings
- Mode toggle: Sandbox (free play) vs Puzzle (guided levels)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/app.js

@@ -1,8 +1,10 @@
 // Entry point — initializes all modules
 import { initRenderer } from './renderer.js';
 import { initEvents } from './events.js';
+import { initPuzzleUI } from './puzzleUI.js';
 
 document.addEventListener('DOMContentLoaded', () => {
     initRenderer();
     initEvents();
+    initPuzzleUI();
 });