fix: Docker-first test suite, UX improvements, and bug fixes (#100)

Dashboard UX:
- Restructure nav from 22 flat links to 6 core + MORE dropdown
- Add mobile nav section labels (Core, Intelligence, Agents, System, Commerce)
- Defer marked.js and dompurify.js loading, consolidate CDN to jsdelivr
- Optimize font weights (drop unused 300/500), bump style.css cache buster
- Remove duplicate HTMX load triggers from sidebar and health panels

Bug fixes:
- Fix Timmy showing OFFLINE by registering after swarm recovery sweep
- Fix ThinkingEngine await bug with asyncio.run_coroutine_threadsafe
- Fix chat auto-scroll by calling scrollChat() after history partial loads
- Add missing /voice/button page and /voice/command endpoint
- Fix Grok api_key="" treated as falsy falling through to env key
- Fix self_modify PROJECT_ROOT using settings.repo_root instead of __file__

Docker test infrastructure:
- Bind-mount hands/, docker/, Dockerfiles, and compose files into test container
- Add fontconfig + fonts-dejavu-core for creative/assembler TextClip tests
- Initialize minimal git repo in Dockerfile.test for GitSafety compatibility
- Fix introspection and path resolution tests for Docker /app context

All 1863 tests pass in Docker (0 failures, 77 skipped).

Co-authored-by: Alexander Payne <apayne@MM.local>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/timmy/backends.py

@@ -194,7 +194,7 @@ class GrokBackend:
     ) -> None:
         from config import settings
 
-        self._api_key = api_key or settings.xai_api_key
+        self._api_key = api_key if api_key is not None else settings.xai_api_key
         self._model = model or settings.grok_default_model
         self._history: list[dict[str, str]] = []
         self.stats = GrokUsageStats()