fix(web): make the 'connection lost' notice self-resolving

The notice promised 'this will clear once it's back', but nothing ever
removed it: the toast auto-hid on a 15s timer (not on recovery) and the
v0.9.0 unified inbox kept every notify() in persistent history, so the
notice lingered there forever.

Model it as a single live status instead: one sticky, transient (never
persisted) notice while the server is unreachable, removed the moment any
request reaches the server again. Adds a 'transient' notification flag
(sticky toast + excluded from history so a reload can't orphan it) and
replaces the throttled-toast helper with a connectivity lost/restored pair.
i18n the strings (errors.offline.*) in EN/HU/DE — they were hardcoded English.

frontend/src/i18n/locales/de/errors.json

@@ -3,6 +3,10 @@
   "generic": "Der Server konnte die Aktion nicht ausführen. Bitte erneut versuchen.",
   "server": "Serverfehler",
   "ok": "OK",
+  "offline": {
+    "title": "Verbindung verloren",
+    "body": "Server nicht erreichbar — er startet möglicherweise gerade neu. Dieser Hinweis verschwindet, sobald die Verbindung wieder steht."
+  },
   "boundary": {
     "title": "Etwas ist schiefgelaufen.",
     "subtitle": "Die App ist auf einen unerwarteten Fehler gestoßen.",