fix(flow-chat): distinguish composer context surface

[bobleer]

Summary

• Follow up on fix(flow-chat): make composer context background opaque #2618 by restoring visual separation between the composer context band and the input surface without reintroducing transparency.
• Mix the opaque panel surface 94% with primary content 6%, which makes the band lighter in dark themes and darker in light themes.
• Strengthen the ChatComposer contract test to require the theme-adaptive mix and reject a transparent background.

Type and Areas

Type: UI/UX regression fix

Areas: Design system, Web UI / FlowChat

Motivation / Impact

Using the panel token directly made the context band visually merge with the input surface. The new opaque mix preserves a distinct hierarchy in both color schemes: approximately #28282b over #1c1c1f in the default dark theme, and #f1f1f1 over #ffffff in the default light theme. No product behavior, persistence, protocol, or localization changes are involved.

Verification

• pnpm run check:web — passed, including Web UI type-check, Appearance contracts, theme color audits, and visual governance.
• pnpm --filter @bitfun/ui run build — passed.
• node --test design-system/packages/ui/tests/chat-composer.test.mjs — passed (4/4).
• git diff --check — passed.

Reviewer Notes

• This is a focused visual follow-up to merged PR fix(flow-chat): make composer context background opaque #2618.
• Remote workspace, remote control, Peer Device Mode, and Detached Dispatch were not exercised separately; this CSS-only shared-component change has 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.
• User-facing strings, docs, and locales are unchanged and not applicable.