#21574 fix(cmux-next): align composer control chrome

Summary

Align cmux NEXT composer control chrome with the shared compact rhythm: location controls keep transparent hover hit areas, chevrons receive the same optical correction, focus rings remain clear, and the model/effort/action spacing stays stable across narrow rows.

Changelog

Changed

• Tightened composer control spacing and chevron alignment while preserving the existing location and New Tab implementations.

Testing

• cd webviews && bun run typecheck — passed; no type errors in 1514 files.
• cd webviews && bun test test/composer-row-fit.test.ts — 3 passed.
• cd webviews && bun test test/composer-row-rhythm.test.ts — browser lane skipped locally because CMUX_BROWSER_TESTS is unset; the suite is intended for CI/Freestyle browser hosts.
• git diff --check — passed.

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Summary by cubic

Aligns the cmux NEXT composer control chrome with the shared compact rhythm so location, model/effort, and action controls draw consistent spacing, hover, and focus behavior.

• Location and composer chip controls keep transparent hover hit areas; only open or selected controls show their state wash.
• Down-chevrons across location and picker controls share the same 0.75px optical correction, now asserted by the row-rhythm test.
• Focus-visible outlines are uniform 2px with a 2px offset (larger under prefers-contrast: more).
• Model/effort row gaps are retuned to 7px/10px so they read as the same optical gap as trailing controls.
• The icons-fit mode keeps the effort chip at its measured 49px width.