[simonasr]

Changing the catch-all browser currently requires opening its rule editor.

This PR adds radio buttons beside browsers for one-click selection. Selection saves immediately without closing the popover or losing focus. Browser names and chevrons still open rule editing; routing rules and macOS default-browser registration remain unchanged.

• Adds a labeled radio group with arrow-key navigation.
• Shows the selected browser as 2 rules · Default.
• Adds keyboard and persistence regression tests and updates the README.

Manually verified selection, Tab/Shift-Tab, Space, and arrow navigation. Spoken VoiceOver output was not tested.

All 27 tests and strict SwiftLint pass. The full formatter check flags pre-existing issues in untouched core files; changed files pass.

Code/config  +127  -32
Tests        +145   -0
Docs           +4   -3