Add utility functions for date formatting, time rounding, and notifications

- Implemented functions to format dates between YYYY-MM-DD and DD.MM.YYYY
- Added functions to get today's date in ISO format
- Created functions to round time to the nearest 15 minutes
- Developed a function to format time as HH:MM
- Added a function to format duration in HH:MM:SS
- Implemented a toast notification system with auto-remove functionality
- Added functions to get day of the week and month names in German

public/js/holidays.js

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+/**
+ * Holiday Functions
+ * German public holidays calculation and checking
+ */
+
+/**
+ * Check if date is weekend
+ */
+function isWeekend(date) {
+  const day = date.getDay();
+  return day === 0 || day === 6; // Sunday or Saturday
+}
+
+/**
+ * Calculate Easter Sunday for a given year (Gauss algorithm)
+ */
+function getEasterSunday(year) {
+  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 new Date(year, month - 1, day);
+}
+
+/**
+ * Get all public holidays for a given year and Bundesland
+ */
+function getPublicHolidays(year, bundesland) {
+  const holidays = [];
+  
+  // Fixed holidays (all states)
+  holidays.push({ date: new Date(year, 0, 1), name: 'Neujahr' });
+  holidays.push({ date: new Date(year, 4, 1), name: 'Tag der Arbeit' });
+  holidays.push({ date: new Date(year, 9, 3), name: 'Tag der Deutschen Einheit' });
+  holidays.push({ date: new Date(year, 11, 25), name: '1. Weihnachtstag' });
+  holidays.push({ date: new Date(year, 11, 26), name: '2. Weihnachtstag' });
+  
+  // Heilige Drei Könige (BW, BY, ST)
+  if (['BW', 'BY', 'ST'].includes(bundesland)) {
+    holidays.push({ date: new Date(year, 0, 6), name: 'Heilige Drei Könige' });
+  }
+  
+  // Internationaler Frauentag (BE, MV since 2023)
+  if (['BE'].includes(bundesland) || (bundesland === 'MV' && year >= 2023)) {
+    holidays.push({ date: new Date(year, 2, 8), name: 'Internationaler Frauentag' });
+  }
+  
+  // Weltkindertag (TH since 2019)
+  if (bundesland === 'TH' && year >= 2019) {
+    holidays.push({ date: new Date(year, 8, 20), name: 'Weltkindertag' });
+  }
+  
+  // Reformationstag (BB, MV, SN, ST, TH, + HB, HH, NI, SH since 2018)
+  const reformationstagStates = ['BB', 'MV', 'SN', 'ST', 'TH'];
+  if (year >= 2018) {
+    reformationstagStates.push('HB', 'HH', 'NI', 'SH');
+  }
+  if (reformationstagStates.includes(bundesland)) {
+    holidays.push({ date: new Date(year, 9, 31), name: 'Reformationstag' });
+  }
+  
+  // Allerheiligen (BW, BY, NW, RP, SL)
+  if (['BW', 'BY', 'NW', 'RP', 'SL'].includes(bundesland)) {
+    holidays.push({ date: new Date(year, 10, 1), name: 'Allerheiligen' });
+  }
+  
+  // Buß- und Bettag (only SN)
+  if (bundesland === 'SN') {
+    // Buß- und Bettag is the Wednesday before November 23
+    let bussbettag = new Date(year, 10, 23);
+    while (bussbettag.getDay() !== 3) { // 3 = Wednesday
+      bussbettag.setDate(bussbettag.getDate() - 1);
+    }
+    bussbettag.setDate(bussbettag.getDate() - 7); // One week before
+    holidays.push({ date: bussbettag, name: 'Buß- und Bettag' });
+  }
+  
+  // Easter-dependent holidays
+  const easter = getEasterSunday(year);
+  
+  // Karfreitag (Good Friday) - 2 days before Easter (all states)
+  const goodFriday = new Date(easter);
+  goodFriday.setDate(easter.getDate() - 2);
+  holidays.push({ date: goodFriday, name: 'Karfreitag' });
+  
+  // Ostermontag (Easter Monday) - 1 day after Easter (all states)
+  const easterMonday = new Date(easter);
+  easterMonday.setDate(easter.getDate() + 1);
+  holidays.push({ date: easterMonday, name: 'Ostermontag' });
+  
+  // Christi Himmelfahrt (Ascension Day) - 39 days after Easter (all states)
+  const ascension = new Date(easter);
+  ascension.setDate(easter.getDate() + 39);
+  holidays.push({ date: ascension, name: 'Christi Himmelfahrt' });
+  
+  // Pfingstmontag (Whit Monday) - 50 days after Easter (all states)
+  const whitMonday = new Date(easter);
+  whitMonday.setDate(easter.getDate() + 50);
+  holidays.push({ date: whitMonday, name: 'Pfingstmontag' });
+  
+  // Fronleichnam (Corpus Christi) - 60 days after Easter (BW, BY, HE, NW, RP, SL, + some communities in SN, TH)
+  if (['BW', 'BY', 'HE', 'NW', 'RP', 'SL'].includes(bundesland)) {
+    const corpusChristi = new Date(easter);
+    corpusChristi.setDate(easter.getDate() + 60);
+    holidays.push({ date: corpusChristi, name: 'Fronleichnam' });
+  }
+  
+  // Mariä Himmelfahrt (Assumption of Mary) - August 15 (BY in some communities, SL)
+  if (['SL'].includes(bundesland)) {
+    holidays.push({ date: new Date(year, 7, 15), name: 'Mariä Himmelfahrt' });
+  }
+  
+  return holidays;
+}
+
+/**
+ * Check if date is a public holiday
+ * Returns the holiday name or null
+ */
+function getHolidayName(date) {
+  const year = date.getFullYear();
+  const holidays = getPublicHolidays(year, currentBundesland);
+  
+  const dateStr = date.toISOString().split('T')[0];
+  const holiday = holidays.find(h => {
+    return h.date.toISOString().split('T')[0] === dateStr;
+  });
+  
+  return holiday ? holiday.name : null;
+}
+
+/**
+ * Check if date is a public holiday
+ */
+function isPublicHoliday(date) {
+  return getHolidayName(date) !== null;
+}
+
+/**
+ * Check if date is weekend or public holiday
+ */
+function isWeekendOrHoliday(date) {
+  return isWeekend(date) || isPublicHoliday(date);
+}