#21391 polish(cmux-next): round composer pills and highlights

Summary

Polish the cmux-next composer to match the rounded pill language in the reference composer surfaces:

• model, effort, and access controls use full capsule radii and a quiet tonal idle fill;
• hover, pressed, open, and keyboard-focus states receive progressively stronger fills/edges;
• add a canonical 400px pill-rail gallery state;
• extend the composer chip browser test to assert pill geometry and state highlights.

This PR is separate from #21363 and does not touch its layout-slot fix.

Validation

• Static composer row and Send/Stop tests pass locally.
• composer-chip-press.test.ts includes the new 400px state; the browser lane requires the hosted Playwright environment.
• git diff --check clean.

The optional effort shortcut hint remains deferred because no existing app-level effort binding is currently exposed; this avoids inventing a new keybinding contract.

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Summary by cubic

Rounds the model, effort, and access controls in the cmux-next composer into tonal pills that match the reference composer surfaces.

• Chips use full capsule radii with a quiet idle fill; hover, pressed, open, and keyboard-focus states step through progressively stronger fills and an inset edge.
• Adds a canonical 400px pill-rail gallery state, including a hover override so the pill fill stays honest in the test gallery.
• Extends the composer chip, quiet-focus, and no-capsules browser tests to assert pill geometry, idle fill, and hover/open/focus highlights.
• Scope the pill menu readiness assertion to the visible menu so the 400px gallery state stays stable across engines.