Dashboard: Sessions nach letzter Chat-Nachricht sortieren statt updated_at

- LEFT JOIN auf messages.created_at (nur type='text')
- COALESCE(last_message_at, updated_at) damit Sessions ohne
  Nachrichten nach ihrem updated_at einsortiert werden

Services/DatabaseService.Sessions.cs

@@ -57,8 +57,17 @@ public partial class DatabaseService
     public async Task<List<SessionWithStats>> GetSessionsWithStats(string userId)
     {
         using var conn = CreateConnection();
-        var sessions = await conn.QueryAsync<SessionRecord>(
-            "SELECT * FROM sessions WHERE user_id = @userId AND is_deleted = 0 ORDER BY updated_at DESC", new { userId });
+        var sessions = await conn.QueryAsync<SessionRecord>(@"
+            SELECT s.* FROM sessions s
+            LEFT JOIN (
+                SELECT c.session_id, MAX(m.created_at) AS last_message_at
+                FROM chains c
+                JOIN messages m ON m.chain_id = c.chain_id
+                WHERE m.type = 'text'
+                GROUP BY c.session_id
+            ) lm ON lm.session_id = s.session_id
+            WHERE s.user_id = @userId AND s.is_deleted = 0
+            ORDER BY COALESCE(lm.last_message_at, s.updated_at) DESC", new { userId });
 
         var results = new List<SessionWithStats>();
         foreach (var s in sessions)