refactor: toten Code entfernen, doppelte Zeit-Formatierung zusammenführen

message_bubble.dart war vollständig unbenutzt (durch message_group.dart
abgelöst). Die zwei identischen _formatTime-Methoden in message_group.dart
zu einer Top-Level-Funktion zusammengeführt - reine Extraktion, keine
Verhaltensänderung. flutter analyze bleibt sauber.

lib/features/chat/message_group.dart

@@ -21,6 +21,12 @@ import 'package:pyramid/utils/gif_favorite_service.dart';
 import 'package:pyramid/widgets/mxc_image.dart';
 import 'package:url_launcher/url_launcher.dart';
 
+String _formatMessageTime(DateTime t) {
+  final h = t.hour.toString().padLeft(2, '0');
+  final m = t.minute.toString().padLeft(2, '0');
+  return '$h:$m';
+}
+
 // Wraps a group of consecutive messages from the same sender
 class MessageGroup extends StatefulWidget {
   final List<Event> events;
@@ -85,7 +91,7 @@ class _MessageGroupState extends State<MessageGroup> {
     final name = sender.calcDisplayname();
     final initial = name.isNotEmpty ? name[0].toUpperCase() : '?';
     final color = _colorFromId(firstEvent.senderId);
-    final time = _formatTime(firstEvent.originServerTs);
+    final time = _formatMessageTime(firstEvent.originServerTs);
     final avatarUri = sender.avatarUrl;
 
     // Build a FLAT column of rows — each message at the same indentation level.
@@ -318,12 +324,6 @@ class _MessageGroupState extends State<MessageGroup> {
     if (id.isEmpty) return colors[0];
     return colors[id.codeUnitAt(0) % colors.length];
   }
-
-  String _formatTime(DateTime t) {
-    final h = t.hour.toString().padLeft(2, '0');
-    final m = t.minute.toString().padLeft(2, '0');
-    return '$h:$m';
-  }
 }
 
 // Shown under a message whose send failed (e.g. offline) — keeps the message
@@ -439,7 +439,7 @@ class _ContinuationMessageState extends State<_ContinuationMessage> {
                 opacity: _hovered ? 1 : 0,
                 duration: const Duration(milliseconds: 120),
                 child: Text(
-                  _formatTime(widget.event.originServerTs),
+                  _formatMessageTime(widget.event.originServerTs),
                   textAlign: TextAlign.center,
                   style: TextStyle(
                     color: widget.pt.fgDim,
@@ -472,11 +472,6 @@ class _ContinuationMessageState extends State<_ContinuationMessage> {
     );
   }
 
-  String _formatTime(DateTime t) {
-    final h = t.hour.toString().padLeft(2, '0');
-    final m = t.minute.toString().padLeft(2, '0');
-    return '$h:$m';
-  }
 }
 
 class _MessageContent extends StatelessWidget {