Fix segmented button borders to render on all four sides

[yxlyx]

Problem

Some segmented/grid-style buttons in the GUI footer do not render borders on all four sides. In the model/controls button row, adjacent buttons appear to lose parts of their outlines, so the controls look uneven and incomplete.

Example from the prompt footer controls:

• model selector (gpt-5.5)
• reasoning effort (High)
• Plan
• Fast
• Ultra

The button grid should consistently show visible borders around every button on all four sides.

Expected behavior

• Every button in segmented/grid button groups has a visible border on all four sides.
• Adjacent buttons may share borders visually, but no side should appear missing.
• Hover/focus/active/selected states should preserve the four-sided button grid appearance.
• The first and last buttons should keep their rounded outer corners while still showing complete borders.

Actual behavior

Some internal and/or edge borders are missing or visually collapsed, making buttons appear to only have partial outlines.

Acceptance criteria

• The prompt footer segmented controls render complete borders around each button.
• Borders remain correct across light/dark themes if applicable.
• Borders remain correct for hover, focus-visible, active, selected, and disabled states.
• The fix applies consistently to reused segmented/button-grid components, not just one hard-coded row.

Notes

This may be caused by adjacent buttons using negative margins, border-overlap rules, missing left/right border classes, or background colors masking shared borders.

[justrach]

@yxlyx Fixed — segmented/grid button borders now render on all four sides (commit b3dd7bf, with tests), shipped in v0.0.166. Thanks for the detailed report!