Add balance adjustments (M8)

- New balance_adjustments table with CRUD store, sync logging, and service methods
- SQL migrations restructured: embed fs.FS from internal/store/migrations/, apply in order via user_version
- WeekService.Balance combines closed-weeks delta + adjustments delta; BalanceSummary breakdown
- Four REST routes: GET/POST /api/balance/adjustments, PUT/DELETE /api/balance/adjustments/{id}
- Dexie schema v2 + sync apply cases for balance_adjustments
- API client: BalanceAdjustment type, balance namespace (list/create/update/delete)
- utils: composeDeltaMs / decomposeDeltaMs helpers + 8 new Vitest tests (19 total, all passing)
- History page: balance card breakdown line + full adjustments section with inline add/edit/delete

internal/store/sync_store.go

@@ -96,6 +96,21 @@ func (s *SyncStore) LogClosedWeek(ctx context.Context, w *domain.ClosedWeek) err
 	return s.log(ctx, "closed_weeks", w.WeekKey, "upsert", string(payload))
 }
 
+// LogBalanceAdjustment appends a balance_adjustment upsert to the sync log.
+func (s *SyncStore) LogBalanceAdjustment(ctx context.Context, a *domain.BalanceAdjustment) error {
+	payload, err := json.Marshal(a)
+	if err != nil {
+		return err
+	}
+	return s.log(ctx, "balance_adjustments", a.ID, "upsert", string(payload))
+}
+
+// LogBalanceAdjustmentDelete appends a balance_adjustment delete to the sync log.
+func (s *SyncStore) LogBalanceAdjustmentDelete(ctx context.Context, id string) error {
+	payload := fmt.Sprintf(`{"id":%q}`, id)
+	return s.log(ctx, "balance_adjustments", id, "delete", payload)
+}
+
 func (s *SyncStore) log(ctx context.Context, entity, entityID, op, payload string) error {
 	version, err := s.nextVersion(ctx)
 	if err != nil {