feat(settings): how many changes wait to sync

The Sync card in Settings says how many changes made on this device
haven't reached the server yet: "3 changes wait to sync", in place of
"Everything synced" until a sync takes them, and beside a failure.

The sync engine can't tell, since the server only says what it has at
the start of each sync, so the app counts: every save asks for a sync,
and a sync that goes through takes the saves made before it started.
The count is kept in the person's folder, so it outlives the app.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_en.arb

@@ -513,6 +513,8 @@
   "syncLastFailed": "Last attempt failed at {time}",
   "@syncLastFailed": {"placeholders": {"time": {"type": "String"}}},
   "syncNow": "Sync now",
+  "syncPending": "{count, plural, =1{1 change waits to sync} other{{count} changes wait to sync}}",
+  "@syncPending": {"placeholders": {"count": {"type": "int"}}},
   "syncTroubleLabel": "Sync needs attention",
   "syncMismatchBanner": "This app can't sync with {server} any more. Update it to keep syncing.",
   "@syncMismatchBanner": {"placeholders": {"server": {"type": "String"}}},