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/db_test.go

@@ -13,7 +13,7 @@ func TestMigration(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer db.Close()
-	tables := []string{"entries", "closed_days", "closed_weeks", "settings_history", "sync_log"}
+	tables := []string{"entries", "closed_days", "closed_weeks", "settings_history", "sync_log", "balance_adjustments"}
 	for _, tbl := range tables {
 		var name string
 		err := db.QueryRow("SELECT name FROM sqlite_master WHERE type='table' AND name=?", tbl).Scan(&name)