docs(tui): note transparent mode assumes a dark terminal

[icn5381]

The transparent override keeps the truecolor foreground palette, which is calibrated for dark terminal backgrounds. State that assumption next to --tui-transparent / DEER_FLOW_TUI_TRANSPARENT so it is discoverable before the mode is flipped on; on a light-background terminal those foregrounds may render with low contrast.

Suggested in the #4631 review.

References #4631

Fixes #

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What changed

Surface area

• Frontend UI — page / component / setting / interaction under frontend/
• Backend API — endpoint / SSE event / request-response shape under backend/app
• Agents / LangGraph — agent node, graph wiring, langgraph.json, or prompt change
• Sandbox — docker/ or sandboxed execution
• Skills — change under skills/
• Dependencies — new/upgraded entry in backend/pyproject.toml or frontend/package.json (say what it buys us)
• Default behavior change — changes existing behavior without the user opting in (default model, default setting, data shape)
• Docs / tests / CI only — no runtime behavior change

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• I've read and understand every line of this change and take responsibility for it — it's not unreviewed AI output.