fix: 2026-06-09 full-codebase audit hardening

Validation: shared NaN-guarded Zod coercion helpers in schemas/common.ts replace
the raw number|string transform idiom across every schema (the root-cause NaN bug
class); emailOrEmpty + lenient isoDateString/timeString.

Security: roles privilege-escalation closed; refresh-token family revocation on
reuse; TOTP uses config params; read endpoints permission-guarded; received-invoices
gross VAT on all paths; orders-pdf custom-items authz.

Concurrency: $queryRaw SELECT...FOR UPDATE locks in ascending-id order (warehouse
confirm/cancel, attendance lockUserRow); uniqueness checks moved into create
transactions (TOCTOU -> 409); deterministic id tiebreak on second-precision
timestamp ordering (plan resolveCell/resolveGrid, warehouse FIFO).

Frontend: Rules-of-Hooks fixed across ~14 pages + PlanCellModal; UTC-date persisted
fields; dashboard invalidation gaps; stale-closure confirm bugs.

Tooling/tests: ESLint flat config (react-hooks/rules-of-hooks = error) + Prettier;
tsconfig.test.json so tsc -b type-checks the tests; removed 3 dead deps; npm audit
fix (8 -> 3). Suite 195 -> 247 (happy-path auth, FIFO oldest-first, flakiness fixes),
isolated on app_test via .env.test with a hard-throw setup guard.

Gates: tsc 0 | build 0 | vitest 247/247 | eslint 0 errors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/utils/czech-holidays.ts

@@ -45,7 +45,15 @@ export function getHolidays(year: number): string[] {
     `${y}-12-26`, // St. Stephen's Day
   ];
 
-  // Easter-based
+  // Easter-based.
+  // TZ NOTE (audit finding addressed): `new Date("YYYY-MM-DD")` parses as UTC
+  // midnight, but the ±2/±1 day shifts below mutate via the LOCAL-time setters
+  // (setDate/getDate) and `localDateStr` reads back in local time. This is only
+  // correct because the app forces `process.env.TZ = "Europe/Prague"` (a
+  // positive UTC offset) in src/config/env.ts — UTC-midnight resolves to the
+  // same local calendar day, so Good Friday/Easter Monday land on the right
+  // dates. Under a NEGATIVE-offset timezone this would read one day early. Safe
+  // given the forced Prague TZ; do not "fix" by mixing UTC and local accessors.
   const easterSunday = getEasterSunday(year);
   const easterDate = new Date(easterSunday);
   const goodFriday = new Date(easterDate);