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/msg-actions.js

@@ -1,21 +1,9 @@
 // ── Timestamp ──
-
-function _formatTimestamp(isoString) {
-  if (!isoString) return '';
-  var d = new Date(isoString);
-  if (isNaN(d.getTime())) return '';
-  var dd = String(d.getDate()).padStart(2, '0');
-  var mm = String(d.getMonth() + 1).padStart(2, '0');
-  var yyyy = d.getFullYear();
-  var hh = String(d.getHours()).padStart(2, '0');
-  var min = String(d.getMinutes()).padStart(2, '0');
-  var ss = String(d.getSeconds()).padStart(2, '0');
-  return dd + '.' + mm + '.' + yyyy + ' ' + hh + ':' + min + ':' + ss;
-}
+// formatTimestamp() → defined in ui-helpers.js
 
 function _addTimestamp(div, timestamp) {
   if (!timestamp) return;
-  var formatted = _formatTimestamp(timestamp);
+  var formatted = formatTimestamp(timestamp);
   if (!formatted) return;
   var span = document.createElement('span');
   span.className = 'msg-timestamp';