Dashboard-Performance: API-Endpoints konsolidieren

Backend:
- GET /api/sessions: Git-Status inline integriert (parallel pro Session)
  - Neue Hilfsmethoden GetGitStatusSummaryAsync (Counts) und
    GetGitStatusDetailedAsync (Dateinamen, für Einzel-Endpoint)
  - GetSessionsWithStats gibt typisierte SessionWithStats zurück
  - Neue Models: SessionWithStats, GitStatusSummary
- GET /api/agents: Skills inline pro Agent integriert
  - SkillService als Parameter in HandleList
  - Skills als Array mit slug/name/description

Frontend:
- Dashboard: 1 paralleler Promise.all statt sequentieller Requests
  - Vorher: agents → sessions → N×git-status → M×agent-skills → cronjobs → shares
  - Nachher: Promise.all([agents, sessions, cronjobs, shares]) (4 Requests)
  - Git-Status direkt aus Session-Response, Skills aus Agent-Response
- Session-Page: Agent-Skills aus inline-Daten statt N×apiGetAgentSkills

Reduziert Dashboard-Requests von ~4+N+M auf 4 (fix).

Services/DatabaseService.cs

@@ -419,13 +419,13 @@ public class DatabaseService
             "SELECT * FROM sessions WHERE user_id = @userId AND is_deleted = 0 ORDER BY updated_at DESC", new { userId });
     }
 
-    public async Task<IEnumerable<object>> GetSessionsWithStats(string userId)
+    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 results = new List<object>();
+        var results = new List<SessionWithStats>();
         foreach (var s in sessions)
         {
             var activeCount = await conn.ExecuteScalarAsync<int>(@"
@@ -440,26 +440,9 @@ public class DatabaseService
                 WHERE c.session_id = @sessionId AND m.type = 'text'",
                 new { sessionId = s.SessionId });
 
-            results.Add(new
+            results.Add(new SessionWithStats
             {
-                s.SessionId,
-                s.Name,
-                s.Description,
-                s.Image,
-                s.AgentId,
-                s.Workspace,
-                s.Context,
-                s.CustomCommands,
-                s.AvatarScale,
-                s.AvatarOffsetX,
-                s.AvatarOffsetY,
-                s.ReviewAgentId,
-                s.MaxReviewIterations,
-                s.FinalizerAgentId,
-                s.PreparerAgentId,
-                s.IsDeleted,
-                s.CreatedAt,
-                s.UpdatedAt,
+                Session = s,
                 MessageCount = activeCount,
                 ArchiveMessageCount = totalCount - activeCount,
                 TotalMessageCount = totalCount,