fix(tui): show keyboard focus in export options - #57154

In Export options (/export), keyboard focus was invisible in places:

• After tabbing to Copy, the screen was identical to the opening state.
• Checked options were drawn in the same colour as the focused one, so you couldn't tell what space or enter would act on.

Fields now use the same styling as the question form (routes/session/form.tsx):

• Label: text.formfield.focused on the focused row, otherwise text.formfield.base, whether or not it's checked.
• Mark (◉ / ○, [x] / [ ]): formfield.focused when focused, formfield.selected when checked, otherwise muted. Only the mark shows what's checked.
• No row backgrounds.

Other changes

• Copy gets the same focus styling as Export (action.primary.focused / .base). Before, it always used a borrowed raised.high background.
• shift+tab moves focus backwards (wrapping), sharing one small helper with tab.

Tests: new test/component/dialog-export-options.test.tsx checks that only the focused label is highlighted (checked ones stay base), that tab ×4 reaches Copy with the focused background, and that shift+tab moves back. test/component and test/ui pass.

Screenshots

Before and after, from the opencode-drive screenshot harness (120×36).

New designs off (opencode theme)

Before	After
Opening state: only the focused Markdown label is orange; checked [x] marks keep their colour
Focus on JSON
Focus on Copy (before: identical to the opening state)
shift+tab from Copy goes back to Include tools

New designs on (new-designs experiment, opencode2 theme)

Here copy and save to file are footer actions with their own keys (c / enter), not tab stops, so tab cycles the options and checkboxes only.

Before	After
Opening state
Focus on JSON
shift+tab moves focus backwards (to Include tools)