Hide YAML frontmatter from editor view

- Add frontmatterHide extension that hides --- ... --- block at top of files
- Shows collapsed --- indicator, reveals on cursor focus
- Place initial cursor after frontmatter so content is visible on load
- Remove frontmatter detection from livePreview (handled by dedicated extension)
- Add Playwright test for frontmatter hiding

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/livePreview.ts

@@ -30,6 +30,7 @@ class HrWidget extends WidgetType {
 
 function buildDecorations(view: EditorView): DecorationSet {
   const decs: Range<Decoration>[] = []
+
   const tree = syntaxTree(view.state)
 
   tree.iterate({