[codex] Hide mobile picker trigger in embedded decks

[natolambert]

Summary

• Detect when a colloquium deck is running inside an iframe and add a colloquium-embedded body class.
• Hide the mobile picker trigger for embedded decks so preview cards do not show the large SLIDES 1 / N pill.
• Also hide the picker trigger in capture mode, matching the rest of the presentation chrome.
• Add the required changelog entry under Unreleased.

Validation

• uv run pytest tests/test_build.py tests/test_parse.py
• uv run python -m colloquium.cli build /Users/nato/Documents/home/me/new-homepage/slides/china-atom-2026/talk.md -o /tmp/china-atom-local-colloquium-check