feat: portal-wide device selector — session-based, all pages now use selected device

- _portal_device_id() reads from session, falls back to DEVICE_NAME env var
- GET/POST /api/v1/portal/select-device for selector data + set
- Dropdown in server base.html topbar, JS populates from API, page reloads on change
- All portal pages (overview, trips, dashcam, DTC, device settings) now use selected device

server/app.py

@@ -74,12 +74,57 @@ login_manager.init_app(app)
 
 
 def _portal_device_id():
-    """Return the device id for the configured device name (single Pi setup)."""
+    """Return the device id for the user's selected device, falling back to
+    the DEVICE_NAME env var default. Selection is stored in the Flask session
+    so it persists across pages without a URL parameter on every link."""
+    # Check session first (user explicitly picked a device)
+    session_id = session.get("selected_device_id")
+    if session_id is not None:
+        dev = db.get_device_by_id(session_id)
+        if dev is not None:
+            return dev["id"]
+        # Selected device was deleted — clear stale session entry
+        session.pop("selected_device_id", None)
+    # Fall back to configured default
     name = os.environ.get("DEVICE_NAME", "yaris-2013")
     dev = db.get_device_by_name(name)
     return dev["id"] if dev else None
 
 
+@app.get("/api/v1/portal/select-device")
+@require_login
+def portal_select_device_state():
+    """Return current selection and available devices for the dropdown."""
+    devices = device_registry.list_devices()
+    configured_name = os.environ.get("DEVICE_NAME", "yaris-2013")
+    for device in devices:
+        device["portal_default"] = device["name"] == configured_name
+    current_id = session.get("selected_device_id")
+    return jsonify(
+        devices=devices,
+        selected_device_id=current_id,
+        configured_device_name=configured_name,
+    )
+
+
+@app.post("/api/v1/portal/select-device")
+@require_login
+def portal_select_device():
+    """Set the active device for the current session."""
+    body = request.get_json(silent=True) or {}
+    device_id = body.get("device_id")
+    if device_id is None:
+        session.pop("selected_device_id", None)
+        session.pop("selected_device_name", None)
+        return jsonify(status="ok", message="Device selection cleared.")
+    dev = db.get_device_by_id(int(device_id))
+    if dev is None:
+        return jsonify(error="Device not found"), 404
+    session["selected_device_id"] = dev["id"]
+    session["selected_device_name"] = dev["device_name"] or dev["name"]
+    return jsonify(status="ok", device_id=dev["id"], device_name=dev["device_name"] or dev["name"])
+
+
 @app.get("/health")
 def health():
     """Unauthenticated liveness check - used by docker-compose healthcheck