Wire up @codemirror/lang-markdown with a custom HighlightStyle to
highlight headings, bold, italic, strikethrough, links, lists,
blockquotes, and inline code in the raw CodeMirror editor. The custom
frontmatter plugin is kept for YAML highlighting; its heading
decoration is removed in favour of the language-based parser.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Adds line numbers, current line highlight, and syntax highlighting
(YAML frontmatter keys/values, --- delimiters, markdown headings).
Extracted useCodeMirror hook and frontmatterHighlight extension.
Preserves wikilink autocomplete, Cmd+S save, and Escape dismiss.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
