Add sync redesign with offline fallback (M9)

- Migration 003: adds logged_at to sync_log for TTL pruning; migrates
  settings_history to UUID TEXT PK with updated_at column
- SyncStore: Prune() deletes rows older than 30d and writes a '_pruned'
  marker at the boundary version; Pull() calls Prune lazily and returns
  ErrSyncStale (410) when the client's since_version is behind the marker
- sync_handler.go: GET /api/sync/pull?since=N; POST /api/sync/push with
  last-updated_at-wins conflict resolution for entries, balance_adjustments,
  settings_history; closed_days/closed_weeks skipped (server-only mutations)
- router.go: passes entryStore, adjustmentStore, settingsStore to SyncHandler
- settings_store.go: UUID PK, updated_at column, Upsert() for push path
- settings_service.go: generates UUID on create, sets updated_at on update
- settings_handler.go: ID params changed from int64 to string
- domain.go: Settings.ID string, Settings.UpdatedAt added
- client.ts: all mutation methods catch TypeError (offline) and fall back
  to Dexie write + outbox enqueue; crypto.randomUUID() for offline creates;
  Settings.id type changed to string
- db.ts: Dexie v3 — settings_history key path changed to string UUID;
  upgrade handler clears table for repopulation via pull
- sync.ts: real pushOutbox to POST /api/sync/push; pullChanges uses GET
  with ?since=N; 410 triggers coldStart() + retry; coldStart() wipes all
  tables and resets last_version
- 4 new Go store tests covering normal pull, stale client, empty prune,
  client-ahead-of-marker; all tests pass (store + service, 19 Vitest)

internal/domain/domain.go

@@ -66,12 +66,13 @@ type ClosedWeek struct {
 
 // Settings holds the effective configuration for a period.
 type Settings struct {
-	ID            int64   `json:"id"`
+	ID            string  `json:"id"`
 	EffectiveFrom string  `json:"effective_from"` // YYYY-MM-DD
 	HoursPerWeek  float64 `json:"hours_per_week"`
 	WorkdaysMask  int     `json:"workdays_mask"` // bits Mon=1..Sun=64
 	Timezone      string  `json:"timezone"`
 	CreatedAt     int64   `json:"created_at"`
+	UpdatedAt     int64   `json:"updated_at"`
 }
 
 // DailyExpectedMs returns the expected milliseconds for a single workday.