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>

Services/DatabaseService.Sessions.cs

@@ -163,6 +163,7 @@ public partial class DatabaseService
         if (body.TryGetProperty("moaLayers", out var mol) && mol.ValueKind == System.Text.Json.JsonValueKind.Number) { sets.Add("moa_layers = @moaLayers"); p.Add("moaLayers", mol.GetInt32()); }
         if (body.TryGetProperty("unityMcpUrl", out var umu)) { sets.Add("unity_mcp_url = @unityMcpUrl"); p.Add("unityMcpUrl", umu.ValueKind == System.Text.Json.JsonValueKind.Null ? null : umu.GetString()); }
         if (body.TryGetProperty("unityMcpToken", out var umt)) { sets.Add("unity_mcp_token = @unityMcpToken"); p.Add("unityMcpToken", umt.ValueKind == System.Text.Json.JsonValueKind.Null ? null : umt.GetString()); }
+        if (body.TryGetProperty("mcpServers", out var mcps)) { sets.Add("mcp_servers = @mcpServers"); p.Add("mcpServers", mcps.ValueKind == System.Text.Json.JsonValueKind.Null ? null : mcps.GetRawText()); }
 
         await conn.ExecuteAsync($"UPDATE sessions SET {string.Join(", ", sets)} WHERE session_id = @sessionId", p);
     }