Add web-based duty schedule bonus calculator

Implements a complete web application for calculating bonus payments
for weekend and holiday duty shifts according to NRW rules.

Features:
- Employee management (add/remove multiple employees)
- Monthly duty scheduling (full and half shifts)
- Automatic NRW holiday detection (2025-2030)
- Bonus calculation with configurable rules
- LocalStorage for data persistence
- Export/Import functionality (JSON)
- Responsive design for desktop and mobile
- No external dependencies

Calculation Rules:
- Qualifying days: Friday, Saturday, Sunday, public holidays, day before holiday
- Minimum threshold: 2.0 qualifying days required
- Deduction: 1.0 qualifying day after threshold reached
- Rates: Normal days 250€, qualifying days 450€
- Half shifts: 50% of respective rate
- No bonus payment if threshold not reached

Technical Stack:
- Vanilla JavaScript (no frameworks)
- HTML5 & CSS3
- LocalStorage API
- Modern, gradient-based UI design

Files:
- webapp/index.html - Main HTML interface
- webapp/styles.css - Responsive styling
- webapp/app.js - Main application logic and UI handling
- webapp/calculator.js - Bonus calculation engine
- webapp/holidays.js - NRW public holidays provider
- webapp/storage.js - LocalStorage data management
- webapp/README.md - Comprehensive documentation

Updated main README.md to include web app in available versions.

webapp/calculator.js

@@ -0,0 +1,176 @@
+/**
+ * Duty Schedule Bonus Calculator
+ * Calculates bonuses based on weekend and holiday duty shifts
+ */
+class BonusCalculator {
+    constructor(holidayProvider) {
+        this.holidayProvider = holidayProvider;
+        this.RATE_NORMAL = 250;  // Normal day rate (not weekend/holiday)
+        this.RATE_WEEKEND = 450; // Weekend/holiday rate
+        this.MIN_QUALIFYING_DAYS = 2.0; // Minimum qualifying days to trigger bonus
+    }
+
+    /**
+     * Check if a date is a qualifying day (weekend or holiday related)
+     * Qualifying days: Friday, Saturday, Sunday, Public Holiday, Day before public holiday
+     * @param {Date} date
+     * @returns {boolean}
+     */
+    isQualifyingDay(date) {
+        const dayOfWeek = date.getDay(); // 0 = Sunday, 5 = Friday, 6 = Saturday
+
+        // Weekend: Friday (5), Saturday (6), Sunday (0)
+        const isWeekend = dayOfWeek === 5 || dayOfWeek === 6 || dayOfWeek === 0;
+
+        // Public holiday
+        const isHoliday = this.holidayProvider.isHoliday(date);
+
+        // Day before public holiday
+        const isDayBeforeHoliday = this.holidayProvider.isDayBeforeHoliday(date);
+
+        return isWeekend || isHoliday || isDayBeforeHoliday;
+    }
+
+    /**
+     * Get day type label for display
+     * @param {Date} date
+     * @returns {string}
+     */
+    getDayTypeLabel(date) {
+        const dayOfWeek = date.getDay();
+        const isHoliday = this.holidayProvider.isHoliday(date);
+        const holidayName = this.holidayProvider.getHolidayName(date);
+        const isDayBefore = this.holidayProvider.isDayBeforeHoliday(date);
+
+        if (isHoliday) {
+            return `Feiertag (${holidayName})`;
+        }
+        if (isDayBefore) {
+            return 'Tag vor Feiertag';
+        }
+        if (dayOfWeek === 5) return 'Freitag';
+        if (dayOfWeek === 6) return 'Samstag';
+        if (dayOfWeek === 0) return 'Sonntag';
+
+        const days = ['Sonntag', 'Montag', 'Dienstag', 'Mittwoch', 'Donnerstag', 'Freitag', 'Samstag'];
+        return days[dayOfWeek];
+    }
+
+    /**
+     * Calculate bonus for a single employee for a given month
+     * @param {Array} duties - Array of duty objects: {date: Date, share: number (1.0 or 0.5)}
+     * @returns {Object} Calculation result
+     */
+    calculateMonthlyBonus(duties) {
+        if (!duties || duties.length === 0) {
+            return this.getEmptyResult();
+        }
+
+        // Separate qualifying and non-qualifying days
+        let qualifyingDays = 0;
+        let normalDays = 0;
+        const dutyDetails = [];
+
+        duties.forEach(duty => {
+            const isQualifying = this.isQualifyingDay(duty.date);
+            const dayType = this.getDayTypeLabel(duty.date);
+
+            if (isQualifying) {
+                qualifyingDays += duty.share;
+            } else {
+                normalDays += duty.share;
+            }
+
+            dutyDetails.push({
+                date: duty.date,
+                share: duty.share,
+                isQualifying: isQualifying,
+                dayType: dayType
+            });
+        });
+
+        // Check if threshold is reached
+        const thresholdReached = qualifyingDays >= this.MIN_QUALIFYING_DAYS;
+
+        let bonus = 0;
+        let normalDaysPaid = 0;
+        let qualifyingDaysPaid = 0;
+        let qualifyingDaysDeducted = 0;
+
+        if (thresholdReached) {
+            // Deduct 1.0 qualifying day
+            qualifyingDaysDeducted = 1.0;
+            qualifyingDaysPaid = Math.max(0, qualifyingDays - qualifyingDaysDeducted);
+            normalDaysPaid = normalDays;
+
+            // Calculate bonus
+            bonus = (normalDaysPaid * this.RATE_NORMAL) + (qualifyingDaysPaid * this.RATE_WEEKEND);
+        }
+
+        return {
+            totalDuties: duties.length,
+            totalDaysWorked: qualifyingDays + normalDays,
+            normalDays: normalDays,
+            qualifyingDays: qualifyingDays,
+            thresholdReached: thresholdReached,
+            qualifyingDaysDeducted: qualifyingDaysDeducted,
+            normalDaysPaid: normalDaysPaid,
+            qualifyingDaysPaid: qualifyingDaysPaid,
+            bonusNormalDays: normalDaysPaid * this.RATE_NORMAL,
+            bonusQualifyingDays: qualifyingDaysPaid * this.RATE_WEEKEND,
+            totalBonus: bonus,
+            dutyDetails: dutyDetails
+        };
+    }
+
+    /**
+     * Calculate bonuses for all employees
+     * @param {Object} employeeDuties - Object with employee names as keys and duty arrays as values
+     * @returns {Object} Results for all employees
+     */
+    calculateAllEmployees(employeeDuties) {
+        const results = {};
+
+        for (const [employeeName, duties] of Object.entries(employeeDuties)) {
+            results[employeeName] = this.calculateMonthlyBonus(duties);
+        }
+
+        return results;
+    }
+
+    /**
+     * Get empty result structure
+     * @returns {Object}
+     */
+    getEmptyResult() {
+        return {
+            totalDuties: 0,
+            totalDaysWorked: 0,
+            normalDays: 0,
+            qualifyingDays: 0,
+            thresholdReached: false,
+            qualifyingDaysDeducted: 0,
+            normalDaysPaid: 0,
+            qualifyingDaysPaid: 0,
+            bonusNormalDays: 0,
+            bonusQualifyingDays: 0,
+            totalBonus: 0,
+            dutyDetails: []
+        };
+    }
+
+    /**
+     * Format currency for display
+     * @param {number} amount
+     * @returns {string}
+     */
+    formatCurrency(amount) {
+        return new Intl.NumberFormat('de-DE', {
+            style: 'currency',
+            currency: 'EUR'
+        }).format(amount);
+    }
+}
+
+// Make it available globally
+window.BonusCalculator = BonusCalculator;