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/domain/domain.go

@@ -109,6 +109,18 @@ func (s *Settings) IsWorkday(wd int) bool {
 	return s.WorkdaysMask&bit != 0
 }
 
+// BalanceAdjustment is a manual corrective entry that adjusts the overall
+// overtime balance without touching week math. DeltaMs is signed: positive
+// credits time, negative debits time.
+type BalanceAdjustment struct {
+	ID          string `json:"id"`           // UUIDv7, client-generated
+	DeltaMs     int64  `json:"delta_ms"`     // signed, non-zero
+	Note        string `json:"note"`         // optional free-text reason
+	EffectiveAt int64  `json:"effective_at"` // unix ms, backdatable
+	CreatedAt   int64  `json:"created_at"`
+	UpdatedAt   int64  `json:"updated_at"`
+}
+
 func popcount(n int) int {
 	count := 0
 	for n != 0 {