Add HTTP server mode with native node:http

Reintroduce server mode (removed in 3014cf9) using node:http instead of
Express — zero new dependencies. Accepts JSON commands via POST to /,
returns JSON responses with CORS support. Screenshot command returns
base64 data when no path is specified.

- Add src/server.ts with BrowserServer class using node:http
- Add -s/--server <port> CLI flag (default 13373)
- Export BrowserServer and startServer from index.ts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

dist/server.d.ts

@@ -4,17 +4,13 @@ export interface ServerOptions extends BrowserOptions {
 }
 export declare class BrowserServer {
     private browser;
-    private app;
     private server;
     private port;
     constructor(options?: ServerOptions);
-    private setupMiddleware;
-    private setupRoutes;
-    private handleCommand;
+    private handleRequest;
     start(): Promise<void>;
     stop(): Promise<void>;
     getPort(): number;
-    getApp(): import("express-serve-static-core").Express;
 }
 export declare function startServer(options?: ServerOptions): Promise<BrowserServer>;
 //# sourceMappingURL=server.d.ts.map