fix: create initial page in BrowserContext sessions

BrowserContext starts with zero pages, so createSession() must call
newPage() after creating the McpContext. Also reorders viewport before
navigation for correct initial layout.

Author: RobertWsp

src/SessionManager.ts

@@ -78,16 +78,18 @@ export class SessionManager {
       contextOptions,
     );
 
-    if (options.url) {
-      const page = context.getSelectedPage();
-      await page.goto(options.url, {waitUntil: 'load'});
-    }
+    // BrowserContext starts with zero pages, so we must create an initial one.
+    const page = await context.newPage();
 
+    // Set viewport before navigation for correct initial layout.
     if (options.viewport) {
-      const page = context.getSelectedPage();
       await page.setViewport(options.viewport);
     }
 
+    if (options.url) {
+      await page.goto(options.url, {waitUntil: 'load'});
+    }
+
     const session: Session = {
       sessionId,
       label: options.label,