Browser homepage for new in-app browser tabs

[jamesvillarrubia]

New tabs in the desktop in-app browser always open on a blank page. Most developers have one page they return to while they work: a local dev server, a GitHub issues list, a Linear board. Today they type that URL into every new tab.

Proposal: add a Browser homepage field to Settings → Integrations → Browser, next to the existing viewport, zoom, appearance, and profile defaults. A tab the user opens without a URL starts there, so every new tab lands on the page they work from. The field defaults to empty, and an empty field behaves exactly as T3 Code does today, so no existing behavior changes. Agent preview_open, clicked links, and discovered ports keep opening their own URLs.

The field accepts any http or https URL and uses the address bar's rules (localhost:3000 becomes http://localhost:3000/). It is one client-local setting with no server or protocol change.

This is a small change, so I put up a PR anyway: #13465