feat(260925-bow): Was-ist-neu-Fenster im Portal-Rahmen

- release-notes.ts: reine Auswahl der Versionsabschnitte aus CHANGELOG.md
- release-notice-actions.ts ('use server'): Abruf und Merken ueber die API
- ReleaseNoticeDialog: barrierefreies Fenster, Eintraege ueber ChangelogView (variant plain)
- ReleaseNoticeHost in AppShell: einmal je Seitenladung, merkt erst beim Schliessen
- Texte releaseNotice in de.json und en.json

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/web/src/messages/en.json

@@ -1291,5 +1291,20 @@
       "notificationSectionTitle": "Notification",
       "platformFeedsNote": "These feeds are maintained by the administration and apply to everyone."
     }
+  },
+  "releaseNotice": {
+    "title": "New in version {version}",
+    "intro": "Tessera has been updated. Here is what changed for you:",
+    "versionHeading": "Version {version}",
+    "section": {
+      "new": "New",
+      "changed": "Improved",
+      "fixed": "Fixed"
+    },
+    "moreVersions": "There are also changes from {count, plural, one {# earlier version} other {# earlier versions}}.",
+    "showAll": "View all changes",
+    "confirm": "Got it",
+    "close": "Close window",
+    "contentLabel": "Changes"
   }
 }