feat: offline read fallback + online status indicator

All GET calls in client.ts now fall back to Dexie when a network error
(TypeError) is caught, so pages render from cached data when the server
is unreachable:

- entries.list()       → db.entries filtered by day_key range
- days.list()          → db.closed_days filtered by day_key range
- weeks.list()         → db.closed_weeks filtered by week_key range
- weeks.balance()      → computed locally from closed_weeks + balance_adjustments
- balance.list()       → db.balance_adjustments ordered by effective_at DESC
- settings.current()   → db.settings_history, latest row with effective_from <= today
- settings.history()   → db.settings_history ordered by effective_from DESC

Day/week close and reopen remain online-only (they require server-side
computation).

Add isOnline store (navigator.onLine + window online/offline events) and
an amber 'Offline — showing cached data' banner in +layout.svelte shown
whenever the store is false.

web/src/lib/stores/online.ts

@@ -0,0 +1,8 @@
+import { writable } from 'svelte/store';
+
+export const isOnline = writable(typeof navigator !== 'undefined' ? navigator.onLine : true);
+
+if (typeof window !== 'undefined') {
+	window.addEventListener('online', () => isOnline.set(true));
+	window.addEventListener('offline', () => isOnline.set(false));
+}