fix: render markdown in multiple-choice survey options

[liuziyi219]

Summary

Survey question titles are rendered through convertMarkdownToHTML, but
multiple-choice option text was interpolated raw. Markdown such as
**bold** therefore displayed literally in radio-button labels and dropdown
options, in both the participant survey view and the experimenter summary view.

Changes

• survey_view.ts (participant view) — render markdown for radio-button
  option labels (regular and image variants) and dropdown options.
• survey_summary_view.ts (experimenter summary view) — same fix for
  radio-button option labels.
• survey_view.scss — add the html-preview mixin to .radio-question
  so the <p> wrapper emitted by the markdown converter does not introduce
  stray margins. .question already includes this mixin.

Option text is now rendered the same way question titles already are
(unsafeHTML(convertMarkdownToHTML(...))). aria-label attributes are
intentionally left as plain text.

Testing

• Ran locally against the Firebase emulator.
• Created a survey stage with a multiple-choice question containing markdown
  (**bold**, _italic_) in both the question title and the options.
• Confirmed the options now render markdown in the participant view,
  consistent with the question title.

Fixes #1097

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[cjqian]

Nice fix, Ziyi! :)

[jimbojw]

@liuziyi219 , you'll need to fill out the Google CLA.

[liuziyi219]

@liuziyi219 , you'll need to fill out the Google CLA.

I have signed. Could you check again plz? Thanks!