Feature: add an in-app Browser option to the new-session + menu

[thevectorguy]

Please add a native browser workspace to MonoCode.

Current behavior: the sidebar/title-bar + action immediately starts a new agent session.

Requested behavior: turn that into a compact chooser:

• New session (keep the current behavior)
• Browser

Choosing Browser should open a browser tab/pane inside MonoCode, with an address field and normal back/forward/reload navigation. It should coexist with agent sessions rather than replacing them, so a user can read a GitLab issue, docs, or a local dev app beside their work.

This should be a native MonoCode surface, not merely open in default browser, so it can remain available as a first-class workspace option.

[thevectorguy]

Closing as a duplicate of #147, which already covers the requested in-app browser workspace, navigation controls, browser tabs, and local-preview flow.