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>

Endpoints/SessionEndpoint.cs

@@ -83,6 +83,7 @@ public static class SessionEndpoint
                 s.PreparerAgentId,
                 s.UnityMcpUrl,
                 s.UnityMcpToken,
+                s.McpServers,
                 s.IsDeleted,
                 s.CreatedAt,
                 s.UpdatedAt,