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/admin/pages/AttendanceBalances.tsx

@@ -347,7 +347,7 @@ export default function AttendanceBalances() {
                     <th>Zbývá (h)</th>
                     <th>Nemoc (h)</th>
                     <th>Fond roku</th>
-                    <th>Odpracováno</th>
+                    <th>Pokryto</th>
                     <th>+/−</th>
                     <th>Akce</th>
                   </tr>