feat: Czech public holidays in work fund calculation

- Created czech-holidays.ts with 11 fixed + 2 Easter-based holidays
- Fund now automatically excludes public holidays (no manual records needed)
- covered = worked + vacation + sick (NOT holidays — already in fund)
- Renamed "Odpracováno" to "Pokryto" (worked + leave = what counts)
- Removed dependency on holiday attendance records per employee

Matches PHP CzechHolidays::getMonthlyWorkFund() logic exactly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/utils/czech-holidays.ts

@@ -0,0 +1,92 @@
+/**
+ * Czech public holidays & work fund calculator.
+ * Port of PHP CzechHolidays class.
+ */
+
+const holidayCache = new Map<number, string[]>();
+
+/** Easter Sunday using the Anonymous Gregorian algorithm */
+function getEasterSunday(year: number): string {
+  const a = year % 19;
+  const b = Math.floor(year / 100);
+  const c = year % 100;
+  const d = Math.floor(b / 4);
+  const e = b % 4;
+  const f = Math.floor((b + 8) / 25);
+  const g = Math.floor((b - f + 1) / 3);
+  const h = (19 * a + b - d - g + 15) % 30;
+  const i = Math.floor(c / 4);
+  const k = c % 4;
+  const l = (32 + 2 * e + 2 * i - h - k) % 7;
+  const m = Math.floor((a + 11 * h + 22 * l) / 451);
+  const month = Math.floor((h + l - 7 * m + 114) / 31);
+  const day = ((h + l - 7 * m + 114) % 31) + 1;
+  return `${year}-${String(month).padStart(2, '0')}-${String(day).padStart(2, '0')}`;
+}
+
+/** All Czech public holidays for a year (11 fixed + 2 Easter-based) */
+export function getHolidays(year: number): string[] {
+  if (holidayCache.has(year)) return holidayCache.get(year)!;
+
+  const y = String(year);
+  const holidays = [
+    `${y}-01-01`, // Den obnovy samostatného českého státu
+    `${y}-05-01`, // Svátek práce
+    `${y}-05-08`, // Den vítězství
+    `${y}-07-05`, // Den slovanských věrozvěstů Cyrila a Metoděje
+    `${y}-07-06`, // Den upálení mistra Jana Husa
+    `${y}-09-28`, // Den české státnosti
+    `${y}-10-28`, // Den vzniku samostatného československého státu
+    `${y}-11-17`, // Den boje za svobodu a demokracii
+    `${y}-12-24`, // Štědrý den
+    `${y}-12-25`, // 1. svátek vánoční
+    `${y}-12-26`, // 2. svátek vánoční
+  ];
+
+  // Easter-based
+  const easterSunday = getEasterSunday(year);
+  const easterDate = new Date(easterSunday);
+  const goodFriday = new Date(easterDate);
+  goodFriday.setDate(goodFriday.getDate() - 2);
+  const easterMonday = new Date(easterDate);
+  easterMonday.setDate(easterMonday.getDate() + 1);
+
+  const fmt = (d: Date) => `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`;
+  holidays.push(fmt(goodFriday));   // Velký pátek
+  holidays.push(fmt(easterMonday)); // Velikonoční pondělí
+
+  holidays.sort();
+  holidayCache.set(year, holidays);
+  return holidays;
+}
+
+/** Check if a date string (YYYY-MM-DD) is a Czech public holiday */
+export function isHoliday(dateStr: string): boolean {
+  const year = parseInt(dateStr.substring(0, 4), 10);
+  return getHolidays(year).includes(dateStr);
+}
+
+/** Business days in a month (Mon-Fri excluding public holidays) */
+export function getBusinessDaysInMonth(year: number, month: number, upToDay?: number): number {
+  const holidays = getHolidays(year);
+  let count = 0;
+  const daysInMonth = new Date(year, month + 1, 0).getDate();
+  const maxDay = upToDay ? Math.min(upToDay, daysInMonth) : daysInMonth;
+
+  for (let day = 1; day <= maxDay; day++) {
+    const date = new Date(year, month, day);
+    const dow = date.getDay();
+    if (dow !== 0 && dow !== 6) {
+      const dateStr = `${year}-${String(month + 1).padStart(2, '0')}-${String(day).padStart(2, '0')}`;
+      if (!holidays.includes(dateStr)) {
+        count++;
+      }
+    }
+  }
+  return count;
+}
+
+/** Monthly work fund in hours (business days × 8) */
+export function getMonthlyWorkFund(year: number, month: number, upToDay?: number): number {
+  return getBusinessDaysInMonth(year, month, upToDay) * 8;
+}