Fix prompt-ending --- rendering last line as a bold heading

[Matteovanypersele]

Many prompts end with a line of dashes (---) used as a courses content delimiter in the dataset templates. In Markdown, a text line immediately followed by --- (with no blank line in between) is a setext heading: the dashes act as an underline and turn the previous line into an <h2>. As a result, the last line of those prompts was rendered large/bold instead of plain text.

This is standard CommonMark behavior, not a parser bug but it wasn't the intended rendering for these prompts.

Fix

Add isolateThematicBreaks in Prompt.tsx, called inside render(). It inserts the missing blank line around lone dash/equals lines, forcing them to render as a horizontal rule (<hr>) instead of underlining the previous line:

• text\n--- → setext heading (bug)
• text\n\n--- → horizontal rule (intended)

Inline dashes (e.g. a---b) are untouched, since the setext rule only applies to a line made entirely of dashes. The fix covers every render path.