Generalize MCP servers, add model refresh button, add sonnet-5

- MCP: replace fixed single Unity field with a generic list of servers
  ({name, url, token?}); dynamic mcp-config + allowlist glob mcp__*<name>*
  per entry; new mcp-servers-ui.js editor (no data migration)
- Models: frontend refresh button wired to existing POST /api/models/refresh,
  reloads optgroups and restores selection
- Model: add sonnet-5 alias + dropdown option (bare "sonnet" stays 4.6)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Models/SessionRecord.cs

@@ -28,6 +28,7 @@ public class SessionRecord
     public int MoaLayers { get; set; } = 2;       // MoA: Anzahl Proposer-Runden vor der Synthese (Fusion ignoriert)
     public string UnityMcpUrl { get; set; }
     public string UnityMcpToken { get; set; }
+    public string McpServers { get; set; }     // JSON-Array benutzerdefinierter MCP-Server [{name,url,token}]
     public string CreatedAt { get; set; }
     public string UpdatedAt { get; set; }
 }