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]>

deploy/readme.md

@@ -179,6 +179,40 @@ Then test from another device:
 - `http://yaris.local`
 - if mDNS is not available on that client, try `http://10.42.0.1`
 
+## Customizing device identity
+
+The Pi displays a username, car model, device name, and avatar image. Instead of editing Python code, edit `config/device.json`:
+
+```bash
+nano /home/yaris/app/yaris-obd/config/device.json
+```
+
+**Example `config/device.json`:**
+
+```json
+{
+  "owner_name": "Your Name",
+  "car_model": "Toyota Yaris 2013",
+  "device_name": "My Yaris OBD",
+  "avatar_path": "img/yaris.png"
+}
+```
+
+- **`owner_name`**: Displayed in topbar (top-right corner of the web UI)
+- **`car_model`**: Displayed in sidebar (left panel, appears next to car icon)
+- **`device_name`**: Reserved for server-side display (Phase 25.5 feature, not yet used)
+- **`avatar_path`**: Avatar image shown in topbar; must exist in `static/img/` (replace `static/img/yaris.png` with your own image, or point to a different file)
+
+After editing, restart the app:
+
+```bash
+sudo systemctl restart yaris-obd.service
+```
+
+**Important**: The SQLite database filename is based on the username. If you change `owner_name` and restart, a new database will be created and the old data will be orphaned at `db/sql/quoije_Yaris2013.db`. Either:
+- Rename the old database file to match the new username, or
+- Delete the old database if you want a fresh start
+
 ## 7) Updating later
 
 ```bash