fix(panel): walk every control on the extensions browser

[StuBehan]

Follows #194, rebased onto main now that's merged.

Same thing you caught on the extension config page, on its sibling. The browser drew a back chevron and, on a failed fetch, a Try again, and the ring never reached either. Both had a key already (Esc and ⌘R) so nothing was unreachable, it just looked half wired next to a page that walks its buttons.

↑↓ now walk the chevron, Try again and the rows. ⏎ does whatever the ring is on. The bar keeps one hint per action and adds ⏎ to the selected one rather than printing a second hint naming it, which is what made the config page say "Back ⏎ · Move · Back Esc" the first time I tried it. The row verb drops off the bar entirely when the ring isn't on a row, since Install or Settings would be naming something the chevron will not do.

The search field is deliberately not a target. Every other control needs ⏎ to reach it; the field is reached by typing, which is what the page already does with any printable key, and a ring you step onto to start typing is a second way to do a thing that needs none.

Seeding order is rows, then Try again, then the chevron. A failed fetch still shows what's installed so there's usually a row to land on, but with nothing installed and nothing published, Try again is the only thing on the page worth pressing.

Checked in the app: chevron ring and ⏎ on it, the row verb appearing and disappearing, and the return landing back on Browse extensions in Settings. The Try again ring is unit tested but I couldn't get GitHub to rate limit me again to eyeball it.