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/reply.js

@@ -24,7 +24,7 @@ function renderReplyPreview() {
   }
 
   const roleLabel = pendingReply.role === 'user' ? 'User' : 'Assistant';
-  const timeLabel = _formatTimestamp(pendingReply.timestamp) || 'Jetzt';
+  const timeLabel = formatTimestamp(pendingReply.timestamp) || 'Jetzt';
   const preview = truncate(pendingReply.text, 150);
 
   container.innerHTML = `<div class="reply-preview-item">