Rework chat workspace UX

[afurm]

Summary

• Problem: the chat workspace duplicated hierarchy, allowed overly wide message rows, kept starters away from the empty state, and pushed context/composer controls into awkward positions on smaller screens.
• Why it matters: chat is the primary agent surface, so conversation identity, readable messages, reachable controls, and fast context access need to work across desktop, tablet, and mobile.
• What changed: reworked the chat card hierarchy, constrained message width, converted the composer into a compact command bar, moved prompt starters into the empty state, replaced duplicated readiness/context copy with a board-context snapshot, added a responsive context Sheet, improved sidebar conversation search/metadata, and rendered proposed action details with shadcn Item composition.
• What did not change: chat storage, server routes, OpenAI request behavior, board data contracts, and persisted conversation semantics.

Closes #40

Change Type

• Bug fix
• Feature
• Security hardening
• Documentation
• Chore / infrastructure

Security Impact

• New permissions or capabilities? No
• Secrets or token handling changed? No
• New or changed network calls? No
• Local storage or memory scope changed? No
• If any answer is Yes, explain the risk and mitigation: N/A

Verification

• npm audit
• npm run typecheck
• npm test
• npm run build
• npm pack --dry-run
• UI screenshot or recording attached, if UI changed

Notes

• Linked issue: Rework chat workspace UX with shadcn composition #40
• Platform tested: macOS local dev server with Browser/Playwright QA at 1440px, 1024px, and 390px.
• Known limitations: screenshots were used for local verification but are not committed or attached to this draft PR.