feat: upgrade MCP to Docker-based architecture (Beta)

Each MCP server can now run as its own Docker container on a dedicated
per-project bridge network, enabling proper isolation and lifecycle
management. SSE transport is removed (deprecated per MCP spec) with
backward-compatible serde alias. Docker socket access is auto-enabled
when stdio+Docker MCP servers are configured.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src/components/layout/Sidebar.tsx

@@ -24,7 +24,7 @@ export default function Sidebar() {
           Projects
         </button>
         <button onClick={() => setSidebarView("mcp")} className={tabCls("mcp")}>
-          MCP
+          MCP <span className="text-[0.6rem] px-1 py-0.5 rounded bg-yellow-500/20 text-yellow-400 ml-0.5">Beta</span>
         </button>
         <button onClick={() => setSidebarView("settings")} className={tabCls("settings")}>
           Settings