feat(settings): auto-save preferences instead of draft + Save

Every Settings change now persists on its own (debounced ~500ms so a slider drag
is one PUT), with a small transient indicator; the draft/dirty-save UI and the
navigation leave-guard are gone. Also unblocks the navigation extraction — there
is no longer a cross-page "unsaved settings" guard to thread through routing.

frontend/src/i18n/locales/en/settings.json

@@ -17,17 +17,9 @@
     "importing": "Importing your Plex watch history…"
   },
   "save": {
-    "unsaved": "Unsaved changes",
     "saving": "Saving…",
     "saved": "Settings saved",
-    "failed": "Couldn't save",
-    "save": "Save",
-    "discard": "Discard"
-  },
-  "unsaved": {
-    "title": "Unsaved changes",
-    "message": "You have unsaved settings. Discard them and leave?",
-    "discard": "Discard changes"
+    "failed": "Couldn't save"
   },
   "appearance": {
     "colorScheme": "Color scheme",