fix: resolve all 28 findings from the 2026-06-12 full audit (TDD-pinned)

Critical (data integrity):
- warehouse inventory confirm: throw (not return) inside $transaction so a
  failed deficit line rolls back the surplus corrective receipt — retries
  no longer accumulate phantom stock
- warehouse issue confirm: validate batches against the COMBINED quantity
  of all lines (duplicate FIFO-resolved lines drove batches negative)
- attendance delete: restore vacation_used/sick_used for the deleted day
  (in-transaction, clamped at 0)

High:
- auth refresh: terminated sessions (replaced_at only) get a plain 401 —
  the theft branch (family revocation) now fires only on replaced_by_hash
- POST /users strips role_id for non-admin callers (mirrors PUT guard)
- issued-order transition flushes unsaved edits via the full save payload
  when dirty; server contract (items+status in one PUT) pinned
- received-invoices list: usePaginatedQuery + pager (rows 26+ unreachable)
- received-invoice dates: nullableIsoDateString + NaN guard before NAS save
  (Czech-format dates corrupted month/year, orphaned NAS files)
- leave approval skips Czech public holidays and books each calendar year's
  hours against its own balance (mirrors createLeave)

Medium/Low (classes):
- 52 Zod caps aligned to DB column widths across 7 schemas (over-cap input
  500ed at Prisma instead of a Czech 400)
- FK pre-validation: projects update + warehouse receipts/issues return
  Czech 400s instead of P2003 500s
- invoice PDF degrades gracefully when the CNB rate is unavailable
  (recap omitted instead of 500 + lost NAS archival)
- date boundaries: local-day filters (warehouse lists/reports, audit-log),
  @db.Date coercion on invoice dates
- plan updateEntry re-checks the per-cell cap (self-excluding)
- {id} tiebreaks on customers/received-invoices/warehouse-items sorts;
  /items honors the client sort param
- htmlToPdf relaunches once when the shared browser died mid-render
- offer number release parses the year from the document number (cross-year
  finalize+delete left permanent sequence gaps)
- trips/vehicles km fields integer-coerced; AI budget regated to
  settings.company|settings.system; Settings System tab no longer clobbers
  Firma numbering patterns; draft invoices hide the dead PDF button;
  dashboard quick-trip invalidates ["vehicles"]; TOTP secret cap 64;
  audit-log + invoice month buckets day-shift fixes

Docs: corrected the stale "Chromium has no CSS margin-box footers" claim
(html-to-pdf.ts + CLAUDE.md — margin boxes render since Chrome 131); audit
report M3 withdrawn accordingly.

~65 new pinning tests; every finding reproduced RED against the real test
DB before its fix. Suite: 58 files / 634 tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/schemas/common.ts

@@ -110,6 +110,26 @@ export const isoDateString = z.preprocess(
     .regex(/^\d{4}-\d{2}-\d{2}$/, "Datum musí být ve formátu YYYY-MM-DD"),
 );
 
+/**
+ * Nullable/optional variant of isoDateString for edit-form date fields:
+ * "" and null normalize to null (the forms clear date inputs with empty
+ * strings), undefined stays undefined (update semantics: field untouched).
+ * Same lenient trailing-time stripping — a local-midnight datetime written
+ * to a @db.Date column would land a day early (Prisma truncates to the UTC
+ * date part).
+ */
+export const nullableIsoDateString = z.preprocess(
+  (v) => {
+    if (v === undefined) return undefined;
+    if (v === null || v === "") return null;
+    return typeof v === "string" ? v.slice(0, 10) : v;
+  },
+  z
+    .string()
+    .regex(/^\d{4}-\d{2}-\d{2}$/, "Datum musí být ve formátu YYYY-MM-DD")
+    .nullish(),
+);
+
 /** HH:MM (24h) time string. Tolerant of a trailing ":ss" seconds component. */
 export const timeString = z.preprocess(
   (v) =>