fix(release): fehlende Versionsnummern + bgUrl-Validierung

Drei Stellen hatten noch '2.0.0' statt '2.0.1': newtab.html About-Sektion,
data.js Export und app.js Backup-Export. FileReader-Upload in settings.js
validiert jetzt bgUrl via isValidBgUrl() (Defense-in-Depth).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/js/data.js

@@ -28,7 +28,7 @@ function initDataButtons() {
   btnExport.addEventListener('click', async () => {
     const widgetData = await Store.get('widgetStates');
     const data = {
-      version: '2.0.0',
+      version: '2.0.1',
       exported: new Date().toISOString(),
       boards,
       settings,