feat: add Grok (xAI) as opt-in premium backend with monetization

- Add GrokBackend class in src/timmy/backends.py with full sync/async
  support, health checks, usage stats, and cost estimation in sats
- Add consult_grok tool to Timmy's toolkit for proactive Grok queries
- Extend cascade router with Grok provider type for failover chain
- Add Grok Mode toggle card to Mission Control dashboard (HTMX live)
- Add "Ask Grok" button on chat input for direct Grok queries
- Add /grok/* routes: status, toggle, chat, stats endpoints
- Integrate Lightning invoice generation for Grok usage monetization
- Add GROK_ENABLED, XAI_API_KEY, GROK_DEFAULT_MODEL, GROK_MAX_SATS_PER_QUERY,
  GROK_FREE config settings via pydantic-settings
- Update .env.example and docker-compose.yml with Grok env vars
- Add 21 tests covering backend, tools, and route endpoints (all green)

Local-first ethos preserved: Grok is premium augmentation only,
disabled by default, and Lightning-payable when enabled.

https://claude.ai/code/session_01FygwN8wS8J6WGZ8FPb7XGV

src/dashboard/templates/base.html

@@ -39,6 +39,7 @@
       <a href="/lightning/ledger" class="mc-test-link">LEDGER</a>
       <a href="/memory" class="mc-test-link">MEMORY</a>
       <a href="/router/status" class="mc-test-link">ROUTER</a>
+      <a href="/grok/status" class="mc-test-link" style="color:#00ff88;">GROK</a>
       <a href="/self-modify/queue" class="mc-test-link">UPGRADES</a>
       <a href="/self-coding" class="mc-test-link">SELF-CODING</a>
       <a href="/hands" class="mc-test-link">HANDS</a>