feat: replace inline save status with toast notifications in settings

Add a standalone toast notification system (no Provider needed) and use it
for settings save feedback. Skip save when fields are empty. Remove save
button on Anilist local user select in favor of auto-save on change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/backoffice/app/(app)/settings/components/AnilistTab.tsx

@@ -228,7 +228,14 @@ export function AnilistTab({
             <div className="flex items-center gap-3">
               <select
                 value={localUserId}
-                onChange={(e) => setLocalUserId(e.target.value)}
+                onChange={(e) => {
+                  const newLocalUserId = e.target.value;
+                  setLocalUserId(newLocalUserId);
+                  handleUpdateSetting("anilist", {
+                    ...buildAnilistSettings(),
+                    local_user_id: newLocalUserId || undefined,
+                  });
+                }}
                 autoComplete="off"
                 className="flex-1 text-sm border border-border rounded-lg px-3 py-2.5 bg-background focus:outline-none focus:ring-2 focus:ring-ring h-10"
               >
@@ -237,9 +244,6 @@ export function AnilistTab({
                   <option key={u.id} value={u.id}>{u.username}</option>
                 ))}
               </select>
-              <Button onClick={() => handleUpdateSetting("anilist", buildAnilistSettings())} disabled={!localUserId}>
-                {t("common.save")}
-              </Button>
             </div>
           </div>
         </CardContent>