button: Show keyboard focus on borderless variants when focus_ring is off

[madcodelife]

Closes #3298

Problem

With Theme::focus_ring = false, focus_ring_style only tinted the element's border with theme.ring. Button only has a border width on the Default variant and with .outline(), so Ghost / Text / Link and the filled variants (Primary, Secondary, Danger, …) showed no keyboard focus at all, even though they are Tab stops.

Fix

focus_ring_style, when the outer ring is off:

• Element has a border (Default / outline buttons, Input, Checkbox, Radio, Switch track, Select, …): unchanged, the border is tinted.
• Element has no border: an absolute child at inset 0 draws a 1px ring-coloured border, with the element's own corner radii.
  • No layout change: no border width is added and the child is absolutely positioned.
  • It can't be clipped: it sits inside the element's bounds.
  • Radius: with no border or padding between them, the element's radius is already concentric with its edge. Theme radius 0 gives a square ring.
  • Only on keyboard focus: Button still calls prevent_default on mouse down, so a click never focuses it and never shows the ring.

1px matches the width of the tinted border the bordered variants already get, so every variant reads the same. Bordered elements keep the tinted border rather than switching to the inner ring: the tinted border already sits at the edge, and an extra inner line would double it.

focus_ring = true is unchanged (outer 3px ring).

The helper change also covers the other borderless callers of focus_ring_style (questionnaire options, carousel, time field). They now show the inner ring too when focus_ring is off.

Refactor: the border-width / corner-radius reads in focus_ring were moved into small private helpers so the inner ring reuses them. No behaviour change there.

Tests

Two new Metal pixel tests in crates/kit/tests/rendering.rs (main thread, harness = false). They render Ghost (icon), Text, Link and Primary buttons with focus_ring = false:

• borderless_buttons_show_keyboard_focus_inside_when_focus_ring_is_off: moves focus with Tab through Root (focus_next for the first stop, since Root's binding needs something inside it focused). For each button it asserts: no ring pixels while unfocused, ring pixels along the top edge inside its bounds when focused, and zero changed pixels outside its bounds.
• clicking_a_button_draws_no_focus_ring: clicking each button leaves focus empty and draws no ring pixels.

Without the fix, the first test fails on ghost ("a keyboard-focused ghost button must draw a ring inside its bounds").

cargo test -p gpui-kit --features test-support,component,assets --test rendering
cargo test -p gpui-component
cargo clippy -p gpui-component -p gpui-kit --all-targets -- -D warnings