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/store/migrations/003_sync_settings_uuid.sql

@@ -0,0 +1,27 @@
+-- +migrate Up
+
+-- 1. Add logged_at to sync_log for TTL-based pruning.
+ALTER TABLE sync_log ADD COLUMN logged_at INTEGER NOT NULL DEFAULT 0;
+
+-- 2. Migrate settings_history to UUID TEXT primary key and add updated_at.
+ALTER TABLE settings_history RENAME TO settings_history_old;
+
+CREATE TABLE settings_history (
+  id              TEXT PRIMARY KEY,
+  effective_from  TEXT NOT NULL,
+  hours_per_week  REAL NOT NULL,
+  workdays_mask   INTEGER NOT NULL DEFAULT 31,
+  timezone        TEXT NOT NULL DEFAULT 'UTC',
+  created_at      INTEGER NOT NULL,
+  updated_at      INTEGER NOT NULL
+);
+
+INSERT INTO settings_history (id, effective_from, hours_per_week, workdays_mask, timezone, created_at, updated_at)
+SELECT lower(hex(randomblob(16))), effective_from, hours_per_week, workdays_mask, timezone, created_at, created_at
+FROM settings_history_old;
+
+DROP TABLE settings_history_old;
+
+-- +migrate Down
+-- (intentionally left minimal; restoring integer PK requires recreating the table again)
+ALTER TABLE sync_log DROP COLUMN logged_at;