feat(settings): editable model library paths for standalone mode

Standalone users previously had to hand-edit settings.json to configure
primary folder_paths. Add a standalone-only Model Paths section to the
settings modal:

- Backend exposes standalone_mode, folder_paths (with template placeholder
  values filtered out) and a data-driven folder_path_schema derived from
  OTHER_MODEL_FOLDER_SUBTYPES via GET /api/lm/settings
- The new section renders multi-path editors per model type from the
  schema, with inline enable_other_models / sub-type controls so other
  model types are configured without leaving the tab
- Persistent restart-required cues after a save: nav dot, inline notice
  and a global banner (unique id per change so dismissals don't mute
  future reminders)
- The missing-model-paths startup banner and the Other Models no-paths
  empty state now deep-link into the new section instead of pointing at
  settings.json

tests/utils/test_constants.py

@@ -42,4 +42,36 @@ class TestIsEmptyPlaceholderHash:
 
     def test_rejects_non_strings(self):
         assert not is_empty_placeholder_hash(None)
-        assert not is_empty_placeholder_hash(123)
+        assert not is_empty_placeholder_hash(123)
+
+class TestFolderPathSchema:
+    def test_core_keys_first_in_canonical_order(self):
+        from py.utils.constants import CORE_FOLDER_PATH_KEYS, folder_path_schema
+
+        schema = folder_path_schema()
+        core = [entry for entry in schema if entry["category"] == "core"]
+
+        assert [entry["key"] for entry in core] == CORE_FOLDER_PATH_KEYS
+        assert all(entry["sub_type"] is None for entry in core)
+        assert schema[: len(core)] == core
+
+    def test_other_entries_derive_from_subtypes_table(self):
+        from py.utils.constants import OTHER_MODEL_FOLDER_SUBTYPES, folder_path_schema
+
+        schema = folder_path_schema()
+        other = {entry["key"]: entry for entry in schema if entry["category"] == "other"}
+
+        assert set(other) == set(OTHER_MODEL_FOLDER_SUBTYPES)
+        for folder_key, sub_type in OTHER_MODEL_FOLDER_SUBTYPES.items():
+            assert other[folder_key]["sub_type"] == sub_type
+
+    def test_text_encoder_exposes_both_folder_keys(self):
+        from py.utils.constants import folder_path_schema
+
+        text_encoder_keys = [
+            entry["key"]
+            for entry in folder_path_schema()
+            if entry["sub_type"] == "text_encoder"
+        ]
+
+        assert text_encoder_keys == ["text_encoders", "clip"]