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/storage.js

@@ -0,0 +1,231 @@
+/**
+ * Data Storage Manager
+ * Manages employee and duty data using localStorage
+ */
+class DataStorage {
+    constructor() {
+        this.STORAGE_KEY_EMPLOYEES = 'dienstplan_employees';
+        this.STORAGE_KEY_DUTIES = 'dienstplan_duties';
+    }
+
+    /**
+     * Get all employees
+     * @returns {Array} Array of employee names
+     */
+    getEmployees() {
+        const data = localStorage.getItem(this.STORAGE_KEY_EMPLOYEES);
+        return data ? JSON.parse(data) : [];
+    }
+
+    /**
+     * Save employees list
+     * @param {Array} employees - Array of employee names
+     */
+    saveEmployees(employees) {
+        localStorage.setItem(this.STORAGE_KEY_EMPLOYEES, JSON.stringify(employees));
+    }
+
+    /**
+     * Add a new employee
+     * @param {string} employeeName
+     * @returns {boolean} Success status
+     */
+    addEmployee(employeeName) {
+        const employees = this.getEmployees();
+
+        if (employees.includes(employeeName)) {
+            return false; // Already exists
+        }
+
+        employees.push(employeeName);
+        this.saveEmployees(employees.sort());
+        return true;
+    }
+
+    /**
+     * Remove an employee and all their duties
+     * @param {string} employeeName
+     */
+    removeEmployee(employeeName) {
+        // Remove from employees list
+        const employees = this.getEmployees();
+        const filtered = employees.filter(e => e !== employeeName);
+        this.saveEmployees(filtered);
+
+        // Remove all duties for this employee
+        const allDuties = this.getAllDuties();
+        delete allDuties[employeeName];
+        this.saveAllDuties(allDuties);
+    }
+
+    /**
+     * Get all duties data (all employees, all months)
+     * @returns {Object} Object with structure: {employeeName: {year-month: [duties]}}
+     */
+    getAllDuties() {
+        const data = localStorage.getItem(this.STORAGE_KEY_DUTIES);
+        return data ? JSON.parse(data) : {};
+    }
+
+    /**
+     * Save all duties data
+     * @param {Object} duties
+     */
+    saveAllDuties(duties) {
+        localStorage.setItem(this.STORAGE_KEY_DUTIES, JSON.stringify(duties));
+    }
+
+    /**
+     * Get duties for a specific employee and month
+     * @param {string} employeeName
+     * @param {number} year
+     * @param {number} month (1-12)
+     * @returns {Array} Array of duty objects
+     */
+    getDutiesForMonth(employeeName, year, month) {
+        const allDuties = this.getAllDuties();
+        const monthKey = `${year}-${String(month).padStart(2, '0')}`;
+
+        if (!allDuties[employeeName] || !allDuties[employeeName][monthKey]) {
+            return [];
+        }
+
+        // Convert date strings back to Date objects
+        return allDuties[employeeName][monthKey].map(duty => ({
+            ...duty,
+            date: new Date(duty.date)
+        }));
+    }
+
+    /**
+     * Save duties for a specific employee and month
+     * @param {string} employeeName
+     * @param {number} year
+     * @param {number} month (1-12)
+     * @param {Array} duties - Array of duty objects
+     */
+    saveDutiesForMonth(employeeName, year, month, duties) {
+        const allDuties = this.getAllDuties();
+        const monthKey = `${year}-${String(month).padStart(2, '0')}`;
+
+        if (!allDuties[employeeName]) {
+            allDuties[employeeName] = {};
+        }
+
+        // Convert Date objects to strings for storage
+        allDuties[employeeName][monthKey] = duties.map(duty => ({
+            ...duty,
+            date: duty.date.toISOString()
+        }));
+
+        this.saveAllDuties(allDuties);
+    }
+
+    /**
+     * Add a duty for an employee
+     * @param {string} employeeName
+     * @param {number} year
+     * @param {number} month (1-12)
+     * @param {Date} date
+     * @param {number} share (1.0 or 0.5)
+     */
+    addDuty(employeeName, year, month, date, share) {
+        const duties = this.getDutiesForMonth(employeeName, year, month);
+
+        // Check if duty already exists for this date
+        const existingIndex = duties.findIndex(d =>
+            d.date.toDateString() === date.toDateString()
+        );
+
+        if (existingIndex >= 0) {
+            // Update existing duty
+            duties[existingIndex].share = share;
+        } else {
+            // Add new duty
+            duties.push({ date, share });
+        }
+
+        // Sort by date
+        duties.sort((a, b) => a.date - b.date);
+
+        this.saveDutiesForMonth(employeeName, year, month, duties);
+    }
+
+    /**
+     * Remove a duty
+     * @param {string} employeeName
+     * @param {number} year
+     * @param {number} month (1-12)
+     * @param {Date} date
+     */
+    removeDuty(employeeName, year, month, date) {
+        const duties = this.getDutiesForMonth(employeeName, year, month);
+        const filtered = duties.filter(d =>
+            d.date.toDateString() !== date.toDateString()
+        );
+        this.saveDutiesForMonth(employeeName, year, month, filtered);
+    }
+
+    /**
+     * Get all duties for all employees in a specific month
+     * @param {number} year
+     * @param {number} month (1-12)
+     * @returns {Object} Object with employee names as keys
+     */
+    getAllEmployeeDutiesForMonth(year, month) {
+        const employees = this.getEmployees();
+        const result = {};
+
+        employees.forEach(employee => {
+            result[employee] = this.getDutiesForMonth(employee, year, month);
+        });
+
+        return result;
+    }
+
+    /**
+     * Clear all data
+     */
+    clearAll() {
+        localStorage.removeItem(this.STORAGE_KEY_EMPLOYEES);
+        localStorage.removeItem(this.STORAGE_KEY_DUTIES);
+    }
+
+    /**
+     * Export data as JSON
+     * @returns {string} JSON string
+     */
+    exportData() {
+        return JSON.stringify({
+            employees: this.getEmployees(),
+            duties: this.getAllDuties()
+        }, null, 2);
+    }
+
+    /**
+     * Import data from JSON
+     * @param {string} jsonString
+     * @returns {boolean} Success status
+     */
+    importData(jsonString) {
+        try {
+            const data = JSON.parse(jsonString);
+
+            if (data.employees) {
+                this.saveEmployees(data.employees);
+            }
+
+            if (data.duties) {
+                this.saveAllDuties(data.duties);
+            }
+
+            return true;
+        } catch (e) {
+            console.error('Import failed:', e);
+            return false;
+        }
+    }
+}
+
+// Make it available globally
+window.DataStorage = DataStorage;