fix(app): say it out loud when wake-ups do not work

Registration failures from the distributor were swallowed, so push looked
enabled while nothing arrived. Without wake-ups Android suspends the app
and messages only show up once the user opens it — which looks like broken
delivery even though the relay has them queued.

The failure is now remembered and surfaced in Settings, and the chat list
carries a dismissible line explaining the state.

app/lib/l10n/app_en.arb

@@ -271,6 +271,9 @@
   "pushModeFcm": "Google (FCM)",
   "pushModeFcmDesc": "Reliable without an extra app. The device push token goes to Google; message content never does.",
   "pushSetupFailed": "Setting up push failed: {error}",
+  "pushHintOff": "Messages arrive only when you open Šepot — wake-ups aren’t set up.",
+  "pushHintFailed": "Wake-ups aren’t working; you’ll see messages only after opening Šepot.",
+  "pushRegistrationFailed": "Wake-ups aren’t arriving — the distributor refused registration. Try turning push off and on again.",
   "pushNoDistributorTitle": "No distributor",
   "pushNoDistributorBody": "For de-Googled notifications, install a UnifiedPush distributor such as ntfy (F-Droid or Play). Then turn push on again.",
   "commonOk": "OK",