feat(chat): a11y improvements and small-phone breakpoint

[maxnoller]

Summary

Extracted from #323 as an independent PR. Touches only the chat page styles, message actions, and one test — no overlap with the Button extraction (#358) or the pending React 19 refactor.

Accessibility

• aria-label on copy/edit/regenerate/branch-nav buttons and edit textarea
• `.messageBlock:focus-within` reveals message actions for keyboard users
• focus-visible outlines on composer input, attach, and send buttons
• `.composer:focus-within` border highlight
• `@media (prefers-reduced-motion: reduce)` disables thinking-dot and sidebar slide-in animations

Mobile / responsive

• `@media (max-width: 480px)` small-phone breakpoint (wider bubbles, tighter padding, smaller empty-state heading)
• Slash suggestions capped at `max-height: 40vh` below 900px

Bug fix

• Mobile sidebar overlay (`.sidebarOpen`) was `inset: 0` which stretched the 280px panel across the viewport and blocked backdrop clicks. Replaced with explicit `top/left/bottom`.

Test plan

• `vitest run` — 183/183 console tests pass
• `tsc --noEmit` clean
• Tab through chat message actions with keyboard — action row appears on focus
• Enable prefers-reduced-motion in devtools — thinking dots and sidebar animation stop
• Resize <480px — bubbles are wider, padding tighter
• Open mobile sidebar, click backdrop — closes

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[maxnoller]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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