fix(flow-chat): make composer context background opaque

[bobleer]

Summary

• Replace the translucent ChatComposer context-band background with the opaque panel surface token.
• Keep the component token metadata aligned with the implementation.
• Add a focused contract assertion so the context band cannot regress to a translucent surface token.

Type and Areas

Type: UI/UX regression fix

Areas: Design system, Web UI / FlowChat

Motivation / Impact

The extension band above the conversation input no longer reveals the scene behind it, giving workspace, branch, and worktree context a stable background. There are no product-logic, persisted-data, protocol, or localization changes.

Verification

• pnpm run check:web — passed, including Web UI type-check, Appearance contracts, theme color audits, and theme visual governance.
• node --test design-system/packages/ui/tests/chat-composer.test.mjs — passed (4/4).
• git diff --check — passed.
• pnpm --filter @bitfun/ui run test — package build and ChatComposer tests passed; the broader suite still exits on the existing unrelated AskUser registry allowed-token assertion.

Reviewer Notes

• Base branch is 1.0.0-explore because main does not yet contain the ChatComposer implementation changed here.
• Remote workspace, remote control, Peer Device Mode, and Detached Dispatch were not exercised separately; this is a CSS-only shared-component change with no behavioral or transport impact.
• AI-assisted contribution. Testing level: fully tested for the scoped UI change.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are unchanged and not applicable.