fix(web): restore chart keyboard access

[xingkaixin]

Summary

• add a shared horizontal roving-listbox layer to canvas bar and donut charts
• restore keyboard-driven hover details, live announcements, and Escape clearing across all three dashboard charts
• name the Agents and model breakdown cards as accessible regions
• cover daily, agent, and model keyboard navigation with component tests

Testing

• pnpm lint
• pnpm format:check
• pnpm build
• pnpm test
• local browser smoke: ArrowLeft/ArrowRight, End, Escape, live summaries, tooltip state, and focus outline