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+# CLAUDE.md - AI Assistant Guide for Dienstplan-Pro
+
+## Project Overview
+
+**Dienstplan-Pro** is a German-language Progressive Web App (PWA) for calculating bonus payments for weekend and holiday duty shifts according to NRW (Nordrhein-Westfalen) regulations. The application is designed for healthcare or similar organizations that need to track employee on-call duties and calculate corresponding bonuses.
+
+**Primary Language**: German (UI, comments, and documentation)
+**Tech Stack**: Vanilla JavaScript, HTML5, CSS3, LocalStorage API
+**Deployment**: Docker container with Node.js `serve`, designed for Railway hosting
+
+## Architecture
+
+### File Structure
+
+```
+Dienstplan-Pro/
+├── index.html          # Main HTML entry point with tab-based UI
+├── app.js              # Main application class (DienstplanApp) - UI management
+├── calculator.js       # BonusCalculator class - core business logic
+├── holidays.js         # HolidayProvider class - NRW holiday data 2025-2030
+├── storage.js          # DataStorage class - LocalStorage persistence
+├── styles.css          # All CSS styles (responsive, gradient theme)
+├── sw.js               # Service Worker for PWA offline support
+├── manifest.json       # PWA manifest configuration
+├── test-suite.js       # Comprehensive test runner with assertions
+├── test.html           # Browser-based test interface
+├── Dockerfile          # Production deployment configuration
+├── README.md           # User documentation (German)
+└── TEST_GUIDE.md       # Testing documentation (German)
+```
+
+### Module Responsibilities
+
+| Module | Class | Purpose |
+|--------|-------|---------|
+| `app.js` | `DienstplanApp` | UI orchestration, event handling, user interactions |
+| `calculator.js` | `BonusCalculator` | Bonus calculation logic, day type classification |
+| `holidays.js` | `HolidayProvider` | NRW public holiday lookup (2025-2030) |
+| `storage.js` | `DataStorage` | LocalStorage CRUD operations, import/export |
+
+### Global Dependencies
+
+All classes are attached to `window` for cross-module access:
+- `window.DienstplanApp` (instantiated as global `app`)
+- `window.BonusCalculator`
+- `window.HolidayProvider`
+- `window.DataStorage`
+
+Script loading order in `index.html` is critical:
+1. `holidays.js`
+2. `calculator.js`
+3. `storage.js`
+4. `app.js`
+
+## Business Logic
+
+### Qualifying Days (WE/Feiertag)
+
+A day is "qualifying" (eligible for higher bonus rate) if ANY of:
+- **Weekend**: Friday (5), Saturday (6), or Sunday (0)
+- **Public Holiday**: Any NRW state holiday
+- **Day Before Holiday**: The calendar day preceding a public holiday
+
+### Bonus Calculation Rules
+
+```
+Constants:
+- RATE_NORMAL = 250€  (normal weekday rate)
+- RATE_WEEKEND = 450€ (qualifying day rate)
+- MIN_QUALIFYING_DAYS = 2.0 (threshold)
+- DEDUCTION_AMOUNT = 2.0 (deducted from qualifying days)
+
+Algorithm:
+1. Count qualifying days (Friday, Sat, Sun, holidays, day-before-holiday)
+2. Count normal days (Mon-Thu, not holiday-related)
+3. If qualifyingDays < 2.0: NO BONUS (total = 0€)
+4. If qualifyingDays >= 2.0:
+   - Deduct 2.0 from qualifying days (Friday priority)
+   - Bonus = (normalDays × 250€) + (remainingQualifyingDays × 450€)
+```
+
+### Friday Priority Deduction
+
+When deducting the 2.0 qualifying days, Fridays are deducted first before Saturday/Sunday/holidays. This is tracked via `qualifyingDaysFriday` and `qualifyingDaysOther` in the calculator.
+
+### Duty Shares
+
+Duties can be full (1.0) or half (0.5). Half duties count as 0.5 toward all calculations.
+
+## Data Storage
+
+### LocalStorage Keys
+
+```javascript
+STORAGE_KEY_EMPLOYEES = 'dienstplan_employees'  // Array of employee names
+STORAGE_KEY_DUTIES = 'dienstplan_duties'        // Nested duty object
+```
+
+### Data Structure
+
+```javascript
+// Employees: string[]
+["Max Mustermann", "Anna Schmidt"]
+
+// Duties: { employeeName: { "YYYY-MM": duties[] } }
+{
+  "Max Mustermann": {
+    "2025-11": [
+      { "date": "2025-11-22T11:00:00.000Z", "share": 1.0 },
+      { "date": "2025-11-23T11:00:00.000Z", "share": 0.5 }
+    ]
+  }
+}
+```
+
+### Date Handling
+
+- Dates are stored as ISO strings in LocalStorage
+- Use `T12:00:00` when creating dates to avoid timezone edge cases
+- Dates are converted back to Date objects when retrieved
+
+## UI Structure
+
+The app uses a tab-based interface with 4 sections:
+1. **Dienste eintragen** (Enter Duties) - Add/remove shifts
+2. **Berechnung** (Calculation) - Calculate and view bonuses
+3. **Mitarbeiter verwalten** (Manage Employees) - CRUD for employees
+4. **Einstellungen** (Settings) - Export/import, rules info, data clearing
+
+### Toast Notifications
+
+Use `app.showToast(message, type)` where type is:
+- `'success'` - Green
+- `'error'` - Red
+- `'info'` - Blue
+
+## Testing
+
+### Running Tests
+
+1. Serve the app: `python3 -m http.server 8000` or use the Docker container
+2. Open `http://localhost:8000/test.html`
+3. Click "Alle Tests ausführen"
+
+### Test Categories
+
+- **HolidayProvider**: Holiday detection, day-before-holiday
+- **Calculator - Tag-Klassifizierung**: Day type classification
+- **Calculator - Bonusberechnung**: Bonus calculation scenarios
+- **Storage**: CRUD operations, import/export
+- **Edge Cases**: Rounding, performance, leap years
+
+### Adding Tests
+
+```javascript
+runner.test('Test Name', (t) => {
+    const calculator = new BonusCalculator(new HolidayProvider());
+    const duties = [{ date: new Date('2025-11-22T12:00:00'), share: 1.0 }];
+    const result = calculator.calculateMonthlyBonus(duties);
+
+    t.assertEqual(result.totalBonus, 450, 'Expected bonus');
+    t.assertTrue(result.thresholdReached, 'Threshold should be reached');
+    t.assertAlmostEqual(result.qualifyingDays, 1.0, 0.01, 'Qualifying days');
+});
+```
+
+## Development Workflow
+
+### Local Development
+
+```bash
+# Option 1: Python server
+python3 -m http.server 8000
+
+# Option 2: Node.js
+npx http-server -p 8000
+
+# Option 3: Docker
+docker build -t dienstplan-pro .
+docker run -p 3000:3000 -e PORT=3000 dienstplan-pro
+```
+
+### Making Changes
+
+1. All JavaScript is vanilla ES6+ classes
+2. No build step required
+3. Refresh browser to see changes
+4. Run test suite after changes
+
+### Deployment (Railway)
+
+The Dockerfile uses `serve` from npm to serve static files:
+```dockerfile
+FROM node:20-alpine
+RUN npm install -g serve
+WORKDIR /app
+COPY . .
+CMD serve -s . -l tcp://0.0.0.0:${PORT:-3000}
+```
+
+## Code Conventions
+
+### Language
+
+- All user-facing text: **German**
+- Code comments: **German** (existing pattern)
+- Variable/function names: **English** (existing pattern)
+
+### Naming
+
+- Classes: PascalCase (`BonusCalculator`, `DataStorage`)
+- Methods/functions: camelCase (`calculateMonthlyBonus`, `isHoliday`)
+- Constants: UPPER_SNAKE_CASE (`RATE_NORMAL`, `STORAGE_KEY_DUTIES`)
+- DOM IDs: kebab-case (`employee-select-duty`, `calc-month-select`)
+
+### Error Handling
+
+- Storage operations include try/catch with German console.error messages
+- User-facing errors shown via toast notifications
+- Invalid data returns empty arrays/objects rather than throwing
+
+### Date Format
+
+- Display: German locale `toLocaleDateString('de-DE')`
+- Storage: ISO string
+- Internal: JavaScript Date objects with noon time (`T12:00:00`)
+
+## Common Tasks
+
+### Adding a New Holiday
+
+Edit `holidays.js`, add to the appropriate year array:
+```javascript
+{ date: 'YYYY-MM-DD', name: 'Holiday Name' }
+```
+
+### Modifying Calculation Rules
+
+Edit `calculator.js` constants:
+```javascript
+this.RATE_NORMAL = 250;
+this.RATE_WEEKEND = 450;
+this.MIN_QUALIFYING_DAYS = 2.0;
+this.DEDUCTION_AMOUNT = 2.0;
+```
+
+### Adding Export Formats
+
+The app supports multiple export formats in `app.js`:
+- `exportData()` - JSON backup
+- `exportCSV()` - Excel-compatible CSV with BOM
+- `exportBonusReport()` - HTML report for printing/PDF
+- `generateEmailReport()` - Copyable email text
+
+### Extending the UI
+
+1. Add HTML in `index.html` within appropriate tab-content div
+2. Add event listener in `setupEventListeners()` in `app.js`
+3. Implement handler method in `DienstplanApp` class
+4. Style in `styles.css` following existing patterns
+
+## PWA Features
+
+- **Service Worker** (`sw.js`): Caches all assets for offline use
+- **Manifest** (`manifest.json`): Enables "Add to Home Screen"
+- **Cache Version**: `dienstplan-pro-v1` (increment when updating assets)
+
+## Key Gotchas
+
+1. **Timezone Issues**: Always use `T12:00:00` when creating dates from strings to avoid midnight edge cases
+2. **LocalStorage Limits**: ~5MB, sufficient for typical use but no warning when approaching limit
+3. **Float Precision**: Use `assertAlmostEqual` in tests for floating-point comparisons
+4. **Script Order**: `holidays.js` must load before `calculator.js`
+5. **Employee Deletion**: Removes all associated duties automatically
+6. **Duty Updates**: Adding a duty on existing date replaces (not duplicates)
+
+## Git Conventions
+
+Recent commit patterns:
+- `feat:` New features
+- `fix:` Bug fixes
+- Keep commit messages concise and descriptive