Add Excel/CSV export feature for beginner-friendly data export

- Add CSV export button to Dienstplan_Portable.html
- Add CSV export button to webapp (index.html, app.js, styles.css)
- Export includes all duties and monthly summary with calculations
- Use semicolon separator for German Excel compatibility
- Add UTF-8 BOM for proper character encoding
- Add proper CSV escaping for names with special characters
- Add helpful tip explaining CSV compatibility with Excel/LibreOffice/Google Sheets

Co-authored-by: Kenearos <86194771+Kenearos@users.noreply.github.com>

webapp/index.html

@@ -177,7 +177,9 @@
 
                 <div class="settings-section">
                     <h3>Datenexport / Import</h3>
+                    <button id="export-csv-btn" class="btn btn-success">📊 Excel/CSV Export</button>
                     <button id="export-btn" class="btn btn-secondary">Daten exportieren (JSON)</button>
+                    <p class="text-muted" style="margin-top: 10px;">💡 <strong>Tipp:</strong> CSV-Dateien können direkt mit Excel, LibreOffice oder Google Sheets geöffnet werden.</p>
                     <div class="form-group">
                         <label for="import-file">Daten importieren:</label>
                         <input type="file" id="import-file" accept=".json">