test(ui): add chat surface stories

[Astro-Han]

Summary

Adds a Storybook state board for the chat surface under Product/Chat Surface, covering:

• empty chat
• streaming response
• tool activity present
• branched conversation banner
• pending and disabled composer states
• import actions
• long messages at the regular review width
• narrow viewport as its own explicit state

Why

Refs #390. This is PR2 for the Storybook-driven UI polish track: make key ChatView + Composer states reviewable before visual polish.

Scope

Changed:

• Added packages/ui/stories/chat-surface.stories.tsx with local fixture data for ChatView and Composer.
• Added a desktop contract test that pins the required PR2 story exports.

Not included:

• No runtime, streaming, persistence, or app shell behavior changes.
• No CSS or visual polish.
• No full ToolActivity variant matrix; this PR only shows tool activity present inside chat.

Verification

• npm run -w @maka/desktop test
• npm run -w @maka/desktop build-storybook
• npm run typecheck
• Confirmed generated Storybook index includes all 8 product-chat-surface--* stories.

User-facing impact

No production UI behavior change. Storybook only.

Reviewer notes

The worktree required npm install before Storybook verification because the borrowed-dependency bootstrap did not expose the storybook bin in this worktree.