Add testable question engine, streak fixes, mobile case grid

- New question-generation engine (shuffle-bag preposition selection,
  per-preposition sentence frames, genitive noun forms) in src/engine.js
- Streak fixes: local-date handling, lapse display
- Footer copy: "Get in touch"
- 2x2 mobile case grid
- Vitest suite (60 tests) covering the engine

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview --host"
+    "preview": "vite preview --host",
+    "test": "vitest run"
   },
   "dependencies": {
     "react": "^18.3.1",
@@ -14,6 +15,7 @@
   },
   "devDependencies": {
     "@vitejs/plugin-react": "^4.3.4",
-    "vite": "^5.4.11"
+    "vite": "^5.4.11",
+    "vitest": "^4.1.10"
   }
 }