feat(settings): explicit Save/Discard for preferences with dirty tracking

Settings-page prefs (theme/scheme/dark-mode/list-view/perf/hints/font + the
notification settings) were each auto-saved to the server on every toggle via
fire-and-forget savePrefs().catch(() => {}) — silent on failure, and no
positive confirmation on success, so the user had zero feedback either way.

Make them a draft instead: changes apply locally for instant preview but
persist only on an explicit Save (or revert on Discard). App owns the live
draft + the last-saved baseline, computes dirty, and exposes a controller to
the panel. The panel shows a Save/Discard bar with 'Saving…' → 'Settings
saved' (auto-clearing) / 'Couldn't save' feedback. Leaving the page with
unsaved changes prompts a confirm (in-app nav + browser Back), and a
beforeunload guards reload/close. savePrefs is now idempotent so the Save
survives a transient gateway blip; failures surface via the connection-lost
status. Language & sidebar layout stay instant (edited outside this page).

New i18n keys settings.save.* / settings.unsaved.* in EN/HU/DE.

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

@@ -6,6 +6,19 @@
     "sync": "Sync",
     "account": "Account"
   },
+  "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"
+  },
   "appearance": {
     "colorScheme": "Color scheme",
     "display": "Display",