chore(libraries): deprecate multi-library support ahead of removal

Warn at startup (log + dismissible startup message) when settings.json
contains more than one library, marking the feature as deprecated while
keeping it fully functional. Removal is planned for one of the next
releases, at which point all libraries' folder paths will be merged into
a single library. Multiple folder roots within one library remain fully
supported and unaffected.

- SettingsManager: add _warn_if_multi_library() invoked from
  _migrate_to_library_registry
- Settings UI: label the Active Library selector as deprecated and note
  the removal plan in its help text (en.json)
- Docs: add deprecation banners to multi_library_design.md and
  library-switching.md
- Remove dead code: SettingsManager.update_active_library_paths had no
  call sites

py/services/settings_manager.py

@@ -583,6 +583,39 @@ class SettingsManager:
         if changed and initial_bootstrap:
             self._needs_initial_save = True
 
+        self._warn_if_multi_library()
+
+    def _warn_if_multi_library(self) -> None:
+        """Surface a deprecation notice when more than one library is configured."""
+        libraries = self.settings.get("libraries", {})
+        if not isinstance(libraries, Mapping) or len(libraries) <= 1:
+            return
+
+        names = ", ".join(sorted(str(name) for name in libraries))
+        logger.warning(
+            "Multi-library support is deprecated and will be removed in an "
+            "upcoming release. Detected %d libraries in settings.json (%s). "
+            "Multiple folder roots within a single library remain fully "
+            "supported; please consolidate your libraries into one.",
+            len(libraries),
+            names,
+        )
+        self._add_startup_message(
+            code="multi-library-deprecated",
+            title="Multi-library support is deprecated",
+            message=(
+                f"This installation has {len(libraries)} libraries configured in "
+                f"settings.json ({names}). Multi-library support is deprecated and "
+                "will be removed in an upcoming release; at that point all "
+                "libraries' folder paths will be merged into a single library. "
+                "Multiple folder roots within a single library remain fully "
+                "supported, so consider consolidating into one library now."
+            ),
+            severity="warning",
+            actions=self._default_settings_actions(),
+            dismissible=True,
+        )
+
     def _sync_active_library_to_root(self, *, save: bool = False) -> None:
         """Update top-level folder path settings to mirror the active library."""
         libraries = self.settings.get("libraries", {})
@@ -2382,34 +2415,6 @@ class SettingsManager:
         if was_active:
             self._notify_library_change(self.settings["active_library"])
 
-    def update_active_library_paths(
-        self,
-        folder_paths: Mapping[str, Iterable[str]],
-        *,
-        extra_folder_paths: Optional[Mapping[str, Iterable[str]]] = None,
-        default_lora_root: Optional[str] = None,
-        default_checkpoint_root: Optional[str] = None,
-        default_unet_root: Optional[str] = None,
-        default_embedding_root: Optional[str] = None,
-        default_other_roots: Optional[Mapping[str, str]] = None,
-        recipes_path: Optional[str] = None,
-    ) -> None:
-        """Update folder paths for the active library."""
-
-        active_name = self.get_active_library_name()
-        self.upsert_library(
-            active_name,
-            folder_paths=folder_paths,
-            extra_folder_paths=extra_folder_paths,
-            default_lora_root=default_lora_root,
-            default_checkpoint_root=default_checkpoint_root,
-            default_unet_root=default_unet_root,
-            default_embedding_root=default_embedding_root,
-            default_other_roots=default_other_roots,
-            recipes_path=recipes_path,
-            activate=True,
-        )
-
     def _notify_library_change(self, library_name: str) -> None:
         """Notify dependent services that the active library changed."""
         libraries = self.settings.get("libraries", {})