refactor(frontend): dedupliziere JS-Code und fixe Bugs

- fix: _oldestLoadedMessageId → _oldestLoadedSortKey (chat.js, chat-page.js)
- move: formatTimestamp nach ui-helpers.js (war _formatTimestamp in msg-actions.js)
- move: _isReviewMessage nach ui-helpers.js (war in history.js)
- add: _getSubAgentMeta + _createSubAgentHeader in ui-helpers.js
- refactor: stream-ui.js + history.js nutzen _createSubAgentHeader
- refactor: flushText() Hilfsfunktion in _renderReviewGroup (history.js)
- refactor: shared-page.js nutzt _mergeConsecutiveAssistantTexts statt eigener Kopie
- refactor: _initCardMenu + _onCardMenuAction Helper in dashboard.js

-53 Zeilen netto

wwwroot/js/pages/chat-page.js

@@ -352,7 +352,7 @@ registerPage('chat', {
     // State zurücksetzen und History laden
     lastKnownMessageId = 0;
     loadedArchiveCount = 0;
-    _oldestLoadedMessageId = 0;
+    _oldestLoadedSortKey = 0;
     _hasMoreOlderMessages = false;
     isStreaming = false;