feat: implement MathTree MVP — skill tree, workbench, and progress system

- Interactive skill tree with dagre auto-layout and React Flow (15 math+logic nodes)
- Workbench with math input (LTR/RTL toggle), multiple choice, keyboard shortcuts
- Challenge verification engine with retry-based flow (no answer reveal until 3 failures)
- Scratchpad canvas with freehand drawing, text boxes, eraser, colors, and stroke sizes
- "Aprende primero" collapsible explanations on introductory challenges
- XP/level system, daily streaks, 7 achievements, progress persistence via Zustand
- Profile page with stats and achievement gallery
- Sidebar navigation with XP bar and streak badge

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -10,6 +10,7 @@
   },
   "dependencies": {
     "@base-ui/react": "^1.3.0",
+    "@dagrejs/dagre": "^3.0.0",
     "@types/katex": "^0.16.8",
     "@xyflow/react": "^12.10.1",
     "class-variance-authority": "^0.7.1",