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>

.claude/launch.json

@@ -0,0 +1,11 @@
+{
+  "version": "0.0.1",
+  "configurations": [
+    {
+      "name": "beug-dev",
+      "runtimeExecutable": "npm",
+      "runtimeArgs": ["run", "dev", "--", "--port", "5199", "--strictPort"],
+      "port": 5199
+    }
+  ]
+}