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]>

modules/app_car.py

@@ -1,4 +1,21 @@
+import json
+import os
+
 class Car:
     def __init__(self):
-        self.model = "Yaris 2013"
-        self.obd = None
+        # Load from config/device.json if available, else use defaults
+        config = self._load_config()
+        self.model = config.get("car_model", "Yaris 2013")
+        self.device_name = config.get("device_name", "Yaris OBD")
+        self.obd = None
+    
+    def _load_config(self):
+        """Load device config from JSON file, return dict with defaults."""
+        config_path = os.path.join(os.path.dirname(os.path.dirname(__file__)), "config", "device.json")
+        try:
+            if os.path.exists(config_path):
+                with open(config_path, 'r') as f:
+                    return json.load(f)
+        except (IOError, json.JSONDecodeError):
+            pass
+        return {}