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

locales/en.json

@@ -555,8 +555,8 @@
       "cardBlurAmountHelp": "Adjust the blur intensity of the header and footer overlays on model and recipe cards (0 = no blur, 20 = maximum blur)."
     },
     "folderSettings": {
-      "activeLibrary": "Active Library",
-      "activeLibraryHelp": "Switch between configured libraries to update default folders. Changing the selection reloads the page.",
+      "activeLibrary": "Active Library (Deprecated)",
+      "activeLibraryHelp": "Switch between configured libraries to update default folders. Changing the selection reloads the page. Deprecated: multi-library support will be removed in an upcoming release. Multiple folder roots within a single library remain fully supported.",
       "loadingLibraries": "Loading libraries...",
       "noLibraries": "No libraries configured",
       "defaultLoraRoot": "LoRA Root",