Preserve YAML frontmatter line breaks, add option to render frontmatter as plain txt

Features
1

Summary

Add a toggle to render YAML frontmatter as verbatim plain text in preview, while keeping frontmatter parsing functional.

Problem

Joplin’s current frontmatter preview discards original line breaks. YAML lists and multi-line content collapse into one single line, destroying readability of structured metadata. There is no way to fix this formatting distortion.

Proposal

Add a setting under Settings > Notes:

Render YAML frontmatter as verbatim plain text

When enabled:

  1. Frontmatter parsing remains working for plugins.
  2. All original line breaks and indentation inside YAML are preserved in preview.
  3. Frontmatter displays as monospaced plain text without custom styling.
  4. Toggle defaults to OFF for backward compatibility.

Context

We do not request disabling frontmatter parsing. We only want preview to faithfully show the original formatting of the source text.