feat(browser): Dioxus Tabs

#412 in DioxusLabs/blitz — merged 2026-05-01

Summary

• Replace inline about:newtab HTML with a Dioxus-rendered AboutPage component.
• Extract URL/nav helpers from toolbar.rs into a new nav module.

Stubs

Settings, History, and Bookmarks are wired through the menu to Dioxus tabs and currently open a stub placeholder.