fix: image-ref Titel-Bug, CSP, ARIA-Labels, Onboarding i18n

- image-ref.js: .widget-title-text → .widget-title (Titel-Update war kaputt)
- Alle 3 Manifests: explizites content_security_policy (script-src/object-src 'self')
- i18n.js: applyLanguage() setzt aria-label synchron zu title
- newtab.html: 6 Close-Buttons + Search-Submit mit data-i18n-title
- i18n.js: 4 neue Keys (dialog.close, search.submit_title, onboarding.tradecenter_desc)
- onboarding.js: TradeCenter-Beschreibung auf t() umgestellt

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

src/js/image-ref.js

@@ -294,7 +294,7 @@ const ImageRef = {
       // Widget-Titel aktualisieren
       const entry = WidgetManager._widgets.get(imageData.id);
       if (entry) {
-        const titleEl = entry.el.querySelector('.widget-title-text');
+        const titleEl = entry.el.querySelector('.widget-title');
         if (titleEl) titleEl.textContent = text || t('imageref.title');
         entry.state.title = text || t('imageref.title');
       }