fix(assistant): balance composer controls in narrow panels

[chaxus]

Summary

In a narrow AI panel, Send/Stop grew to 40 px while adjacent controls stayed at 32 px. The focused composer also stacked a border and shadow, and busy 320 px panels squeezed the voice language label down to an empty chevron.

• Align desktop composer controls to 32 px, preserve 44 px coarse-pointer execution targets, and use one neutral focus boundary with a 12 px inset.
• Tighten the narrow toolbar through existing tokens and the select component's public spacing hooks so language, Sources, privacy, Stop and Send remain readable in one row.

Follow-up to #274, which merged during the live-preview walkthrough. This PR contains only host CSS, browser regressions and the acceptance record; runtime and shared ChatView contracts are unchanged.

Merge Verdict

[APPROVE] — Spec, code-quality and final post-CI reviews found no outstanding blocking findings. All 19 remote checks passed on db8f6e2e4af388b91aa09fbc4bf030c6fb692a65.

Risk Analysis

Low scope: host styling only. Regression checks preserve touch targets, preparation labels, draft/IME/focus behavior, voice states and queue actions.

Design Decisions

Use the existing 32 px desktop control scale and 44 px touch target. Keep one composer boundary. Use public select CSS variables rather than modifying component internals.

Code Concerns

No outstanding findings in the reviewed increment.

Verification

• 35 integrated browser scenarios passed across ten UI suites after the final CSS change.
• The initial 12 focused scenarios and 10 narrow-language/voice follow-up scenarios passed; both geometry defects were reproduced before fixing them.
• Full lint/TypeScript, formatting/1,511 immutable archive checks and production build passed.
• Actual light/dark/narrow/short/multiline/busy screenshots inspected. English remains fully visible at 320 px while longer language names retain a readable ellipsis lane.

Conclusion basis: Direct preview checks covered drafts/navigation, optional connector feedback, model setting separation and artifact import/edit/version save. Generation and voice regression responses are controlled fixtures, not a live-model or real-account authorization claim. The transferred source tree is identical to the locally verified tree. The deployed preview was rechecked directly: 32 px toolbar controls, 12 px inset, one 1 px focus border without shadow, no horizontal overflow in the 320 px sidebar, and the Chinese draft preserved after source navigation. All 19 latest-head CI checks passed; the final increment was reviewed again afterward. No merge was performed.

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