feat(acp-client): toolbar row with settings, plan, and always-visible cancel

[raphaeltm]

Summary

• Refactors the chat input area to add a toolbar row above the textarea containing the settings button (left), plan button (center-left), and cancel button (far right).
• The cancel button is now always visible — not just when prompting. ACP status reporting can be unreliable with subagents, so users need a way to force-cancel at any time.
• Settings button restyled as a compact pill to match the plan button height.

Validation

• pnpm lint
• pnpm typecheck (via pnpm build)
• pnpm test — 233 tests pass (6 new)
• Mobile and desktop verification: toolbar pills are compact, flex-wrap friendly

UI Compliance Checklist (Required for UI changes)

• Mobile-first layout verified — toolbar uses flex with gap, wraps naturally
• Accessibility checks completed — aria-labels on all toolbar buttons
• Shared UI components used or exception documented — reuses StickyPlanButton, matches pill styling

Exceptions (If any)

N/A

Agent Preflight (Required)

• Preflight completed before code changes

Classification

• external-api-change
• cross-component-change
• business-logic-change
• public-surface-change
• docs-sync-change
• security-sensitive-change
• ui-change
• infra-change

External References

N/A: Frontend-only layout refactor within acp-client package.

Codebase Impact Analysis

• packages/acp-client/src/components/AgentPanel.tsx — toolbar layout refactor
• packages/acp-client/src/components/StickyPlanButton.tsx — remove margin class
• packages/acp-client/src/components/AgentPanel.test.tsx — 6 new tests

Documentation & Specs

N/A: No API or behavioral changes requiring doc updates.

Constitution & Risk Check

• Principle XI: No hardcoded values introduced. All styling uses Tailwind utility classes.
• No security implications — layout-only change.