Enable YAML header rendering for all .md files by default

[MarkShawn2020]

Summary

Flip the factory default of yamlExtensionAll from false to true, so plain .md files get the same structured frontmatter rendering that .rmd / .qmd already get.

Motivation

With the current default, a typical Jekyll / Hugo / Zola / Obsidian file like:

---
title: My post
date: 2025-01-01
tags: [foo, bar]
---

Body...

…is fed to CommonMark as-is. The opening --- gets interpreted as a setext H2 underline, so title: My post becomes a heading, and the remaining keys collapse into a single paragraph. The result looks broken even though the YAML rendering code already exists and works — it's just gated to .rmd / .qmd.

Markdown frontmatter is now the dominant pattern across static-site generators and note-taking apps, so enabling it by default better matches user expectations.

Change

Single line in QLMarkdown/Settings.swift:

-    @objc var yamlExtensionAll: Bool = false
+    @objc var yamlExtensionAll: Bool = true

No logic changes — the existing renderYamlHeader path at Settings+render.swift:105 already handles this cleanly when the flag is on.

Behavior

• New installs: .md frontmatter renders as the existing table layout.
• Existing users: unaffected — their preference is already persisted in UserDefaults.
• Opt-out: Preferences → YAML header popup → ".rmd, .qmd files" restores the old scope. Or switch it off entirely.

Test plan

• Build & install locally, Quick Look a .md file with YAML frontmatter → renders as a table
• Confirm the same file with the preference switched back to ".rmd, .qmd files" renders as before (falls through to CommonMark)
• .rmd / .qmd files unchanged

[hiphapis]

👍

[sbarex]

implemented on release 1.5.4