implement: Phase 25.5 device config from JSON file

- Create config/device.json with owner_name, car_model, device_name, avatar_path
- Update app_account.py to load owner_name and avatar_path from config
- Update app_car.py to load car_model and device_name from config
- Update base.html template to use avatar_path from config instead of hardcoded
- Add graceful fallback to hardcoded defaults if config file missing
- Document in deploy/readme.md and readme.md how to customize device identity
- Update PLAN.md Phase 25.5 to reflect current implementation and future server sync

Benefits:
- No code edits needed to customize Pi (edit JSON file only)
- Offline-first: all values loaded from local file at startup
- Connection-driven sync planned for Phase 25.5a (sync when internet available, not on timer)
- Server becomes source of truth after Phase 25.5a implementation

All 63 tests passing. Database filename still based on owner_name, with migration warning in docs.

Co-authored-by: Copilot <[email protected]>

templates/base.html

@@ -79,7 +79,7 @@
                     </div>
                     <span class="text-muted small d-none d-sm-inline">{{ account.name }}</span>
                     <img class="rounded-circle border" style="width:2rem;height:2rem;object-fit:cover;"
-                         src="{{ url_for('static', filename='img/yaris.png') }}" alt="">
+                         src="{{ url_for('static', filename=account.avatar_path) }}" alt="">
                 </div>
             </div>
         </nav>