Persist the expert_mode preference the frontend already writes

esphome_device_builder/models/preferences.py

@@ -46,6 +46,10 @@ class UserPreferences(DataClassORJSONMixin):
     navigator_visible: bool = True
     yaml_diff_button: bool = False
 
+    # Frontend "Expert Mode" master switch: gates the editor diff view, the
+    # device-navigator search box, and the command-palette YAML content search.
+    expert_mode: bool = False
+
     # Table view settings
     table_page_size: int = 25
     table_column_visibility: dict[str, bool] = field(default_factory=dict)

tests/test_config_controller.py

@@ -747,6 +747,15 @@ async def test_set_prefs_merges_partial_update(tmp_path: Path) -> None:
     assert controller.prefs.snapshot() == result
 
 
+async def test_set_prefs_persists_expert_mode(tmp_path: Path) -> None:
+    """The frontend's Expert Mode toggle round-trips through set/get prefs."""
+    controller = _make_controller(tmp_path)
+
+    result = await controller.set_prefs(expert_mode=True)
+    assert result.expert_mode is True
+    assert (await controller.get_prefs()).expert_mode is True
+
+
 async def test_set_prefs_concurrent_updates_do_not_lose_writes(tmp_path: Path) -> None:
     """Partial updates of distinct fields all survive on the RAM-canonical store.