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/onboarding.js

@@ -215,7 +215,7 @@ const Onboarding = {
         { id: uid(), title: 'Modrinth (Mods)', url: 'https://modrinth.com', desc: '' },
         { id: uid(), title: 'Star Citizen Wiki', url: 'https://starcitizen.tools', desc: '' },
         { id: uid(), title: 'UEX Corp (Trading)', url: 'https://uexcorp.space', desc: '' },
-        { id: uid(), title: 'Hellion TradeCenter', url: 'https://hellion-initiative.online/tradecenter', desc: 'Trade Center f\u00FCr Star Citizen' }
+        { id: uid(), title: 'Hellion TradeCenter', url: 'https://hellion-initiative.online/tradecenter', desc: t('onboarding.tradecenter_desc') }
       ],
       blurred: false
     };