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/migrations/002_balance_adjustments.sql

@@ -0,0 +1,14 @@
+-- +migrate Up
+CREATE TABLE balance_adjustments (
+  id           TEXT PRIMARY KEY,
+  delta_ms     INTEGER NOT NULL,
+  note         TEXT NOT NULL DEFAULT '',
+  effective_at INTEGER NOT NULL,
+  created_at   INTEGER NOT NULL,
+  updated_at   INTEGER NOT NULL
+);
+CREATE INDEX idx_balance_adjustments_effective_at ON balance_adjustments(effective_at);
+
+-- +migrate Down
+DROP INDEX IF EXISTS idx_balance_adjustments_effective_at;
+DROP TABLE IF EXISTS balance_adjustments;