Allow saving integrated browser pages as MHTML

It would be useful to save the current page in the integrated browser as a single MHTML file for offline inspection and archival.

Proposed behavior:

- Add **Save Page As...** to Browser Actions and the Command Palette.
- Bind Ctrl/Cmd+S while the integrated browser is focused.
- Suggest a `.mhtml` filename based on the page title.
- Save the page and its resources to the local filesystem as MHTML.

This complements #333305 and #333307, which add support for opening MHTML files in the integrated browser, enabling a complete save-and-reopen workflow.