fix(dates): @db.Date filters built from local midnight queried the wrong day

Prisma truncates a JS Date used in a WHERE filter on a @db.Date column to
its UTC date part. Under TZ=Europe/Prague a local-midnight boundary
(new Date(y,m,d) = 22:00/23:00Z of the previous day) therefore filtered as
the PREVIOUS calendar date. Same class as the dashboard fix; full sweep of
every @db.Date filter in the codebase. New shared helper
utcMidnightOfLocalDay() in src/utils/date.ts.

Fixed (all previously off by one day):
- attendance.service: getStatus today+month windows; getWorkfund (last day
  of each month was double-counted across months); getPrintData (monthly
  print included prev month's last day); listAttendance (admin month view
  included prev month's last day AND dropped the selected month's last
  day); createAttendance duplicate/overlap validation (checked only the
  PREVIOUS day - same-day duplicates were never caught, neighbors falsely
  rejected)
- invoice-alerts: the 'splatnost za 3 dny' advance alert had NEVER fired
  (due==today+3 was outside the fetched window); window computation
  extracted as computeAlertWindow() + pure tests
- invoices.service: month list/totals filter dropped invoices issued on the
  month's last day; getInvoiceStats month/year bounds; markOverdueInvoices
  boundary; auto paid_date could store yesterday during 00:00-02:00
- received-invoices auto paid_date, issued-orders default order_date: same
  night-window write hazard, now via the helper
- attendance.schema: shift_date hardened to isoDateString (bare YYYY-MM-DD)

+9 regression tests (month boundaries, same-day duplicate, last-day-of-month
invoice in list+totals, alert window).

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

src/utils/date.ts

@@ -8,6 +8,22 @@
  * of JSON serialization (e.g., building lookup keys, shift_date strings).
  */
 
+/**
+ * UTC midnight of the LOCAL calendar day of `d` (default: now).
+ *
+ * Prisma truncates a JS Date used against a `@db.Date` column (filter or
+ * write) to its **UTC** date part. With TZ=Europe/Prague, a local-midnight
+ * Date — `new Date(y, m, d)` — is 22:00/23:00 UTC of the PREVIOUS day, so it
+ * filters/stores as the previous calendar date; and a bare `new Date()`
+ * written to a `@db.Date` column stores yesterday during the 00:00–02:00
+ * Prague window. Always build `@db.Date` values and range boundaries from
+ * `Date.UTC(...)` of the LOCAL calendar components — this helper does exactly
+ * that for "today" (or any reference Date).
+ */
+export function utcMidnightOfLocalDay(d: Date = new Date()): Date {
+  return new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
+}
+
 /** YYYY-MM-DD in local time */
 export function localDateStr(d: Date): string {
   const y = d.getFullYear();