refactor: fix all Low findings from FLAWS_REPORT audit

- Auth: TOTP params from config, JWT error logging, audit log failure
  logging, replaced_by_hash validation on token rotation
- Invoices: remove dead VAT code, consistent PDF permissions,
  WebP magic-byte detection, deduped exchange-rate fetches
- Orders/Offers: multipart limit from config, use paginated() helper,
  payment method from DB in PDF
- Projects: verify project exists before creating note
- Attendance: action_type enum validation, consistent local-time
  shift_date construction, holiday attendance in work fund,
  trips.view permission on last-km query
- Users: paginated() helper usage, remove duplicate dashboard keys,
  parallel currency conversion, single hashToken implementation
- Frontend: memoized customInput, reliable print onload, modal prop
  standardization (isOpen), ConfirmModal type icons, id===0 key
  fallback, Login useCallback, CompanySettings ConfirmModal,
  Attendance timeout cleanup, Dashboard memoization, beforeunload
  dirty-state warnings on Invoice/Offer/Order detail
- Schema: invoice_alert_log timestamp, config/env comment on
  Date.prototype.toJSON override
- Utils: exchange-rate inflight dedup

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/config/env.ts

@@ -5,6 +5,8 @@ dotenv.config();
 process.env.TZ = process.env.TZ || "Europe/Prague";
 
 // Override Date.toJSON so JSON.stringify outputs local time (Europe/Prague).
+// This is intentional and required for PHP migration compatibility: the legacy
+// PHP API returned local Czech times, and the frontend relies on this format.
 // Prisma stores UTC in MySQL DATETIME columns. When reading, it creates
 // JS Date objects with correct UTC internals. The default toJSON() calls
 // toISOString() which returns UTC — this override uses local getters instead,
@@ -50,6 +52,13 @@ export const config = {
 
   totp: {
     encryptionKey: required("TOTP_ENCRYPTION_KEY"),
+    algorithm: (process.env.TOTP_ALGORITHM || "SHA1") as "SHA1",
+    digits: parseInt(process.env.TOTP_DIGITS || "6", 10),
+    period: parseInt(process.env.TOTP_PERIOD || "30", 10),
+    loginTokenExpiryMinutes: parseInt(
+      process.env.LOGIN_TOKEN_EXPIRY_MINUTES || "5",
+      10,
+    ),
   },
 
   nas: {