feat: the app tells when sync fails, and when it is too old

Published apps can't be made to update, so the app now notices when it
no longer matches the server: the server names the oldest app build it
works with (instance.info's minimumAppBuild), asked before the first
sync of each launch, and the sync engine's refusal of differing synced
tables counts the same. Either way the app says "update the app; if it
is up to date, the server needs an update" in a banner on Home and in
the new Sync card in Settings, and what the person does meanwhile stays
on the device.

The Sync card shows where sync stands since the app started (syncing,
everything synced, sync failing, with the last completion and failure)
and offers Sync now. While the last sync failed, a dot sits on the
Profile tab and the Settings cog. One sync that goes through clears it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -267,6 +267,24 @@
   "settingsDeleteTitle": "Delete your account?",
   "settingsDeleteMessage": "Everything you have, your studysets, cards and reviews, is deleted on the server and on all your devices. This can't be undone.",
   "settingsAbout": "About Halkyon Learn",
+  "syncTitle": "Sync",
+  "syncStateSyncing": "Syncing…",
+  "syncStateSynced": "Everything synced",
+  "syncStateNotYet": "Not synced yet",
+  "syncStateFailing": "Sync is failing",
+  "syncStateMismatch": "This app can't sync with {server}",
+  "@syncStateMismatch": {"placeholders": {"server": {"type": "String"}}},
+  "syncMismatchDetail": "Update the app. If it is up to date, the server needs an update. What you do meanwhile stays on this device and syncs afterwards.",
+  "syncLastCompleted": "Last completed at {time}",
+  "@syncLastCompleted": {"placeholders": {"time": {"type": "String"}}},
+  "syncNoRunYet": "No sync has completed since the app started",
+  "syncLastFailed": "Last attempt failed at {time}",
+  "@syncLastFailed": {"placeholders": {"time": {"type": "String"}}},
+  "syncNow": "Sync now",
+  "syncTroubleLabel": "Sync needs attention",
+  "syncMismatchBanner": "This app can't sync with {server} any more. Update it to keep syncing.",
+  "@syncMismatchBanner": {"placeholders": {"server": {"type": "String"}}},
+  "syncMismatchMore": "More",
 
   "tagsReadFailed": "Couldn't read the tags.",
   "tagPickerTitle": "Tags",