Add Finder Quick Actions for exporting Markdown

[mfauzaan]

Summary

Adds four Finder Quick Actions — Create PDF, Create PNG, Create HTML, and a combined Export Markdown — as Action extensions embedded in the app.

Right-click a Markdown file → Quick Actions → pick a format. Each one exports beside the source file the way Finder's own Convert Image does: no destination picker, Finder-style collision naming (full 2.pdf), and the main app never launches. The three format-specific actions still show the preview/font-size sheet, just with the format selector hidden.

The four targets share one source folder and one Info.plist, differing only by FINDER_ACTION_ICON_NAME and the format they're compiled for.

Icons

The icons are generated to match Apple's built-in Finder actions rather than approximated by hand — scripts/generate-action-icons.swift regenerates them and documents the spec, measured off Apple's own Create PDF as rendered in Login Items & Extensions:

Plate	26×26pt, corner radius 8, vertical gradient #B4B4B9 → #8E8E93
Glyph	white, fitted to a centred 14pt box
Canvas	32×32pt — the list draws at natural size and won't upscale

Glyphs are Apple's real SF Symbol outlines (doc, photo, chevron.left.forwardslash.chevron.right, arrow.up.doc), read from NSSymbolImageRep's outlinePath rather than redrawn. doc is the same page-with-top-right-fold silhouette Finder uses for Create PDF, so that one is 1:1 and the others follow the same construction.

Two non-obvious findings, both commented in the generator so they aren't "fixed" later:

• The list does not composite the grey plate. It draws icons exactly as authored — Apple's built-ins and third-party actions alike bake the plate into their own artwork. A bare glyph renders plate-less and undersized.
• The plate is not a flat fill. Sampling a column down Apple's tiles gives ~#B4B4B9 at the top falling to systemGray #8E8E93 about 60% of the way down, then holding — a subtle top-lit sheen. Create PDF and Convert Image measure identically, so it's a system treatment, not per-icon artwork. A flat plate is visibly duller side by side.
• Convert samples to sRGB before comparing against a screen capture. Raw bitmap components come back in the display's space, where #8E8E93 reads as #7B7B81.

PNG at 1x/2x rather than vector PDF: a fill in a CGPDFContext is written untagged and gets reinterpreted, whereas PNG carries an explicit sRGB profile.

Notes for review

Two undocumented Info.plist keys are load-bearing here, verified by removing each and observing the breakage (both now carry comments):

• without NSExtensionServiceAllowsFinderPreviewItem the actions disappear from Finder's list entirely
• without NSExtensionServiceFinderPreviewIconName the rows fall back to the containing app's icon

The changes to MarkdownWebView.swift / MarkdownHTML.swift are small #if EXPORT_ACTION_EXTENSION guards that compile the Mermaid popup and vendor warm-up out of the extension.

On a fresh install users must enable these once in System Settings → General → Login Items & Extensions → Actions. That's unavoidable for third-party Action extensions; Apple's built-ins are Finder-internal and exempt. Worth a first-launch prompt as a follow-up.

Test plan

• Debug build succeeds with all four extensions embedded and validated, rebased on current main
• All four actions appear in Finder's Quick Actions list
• Real exports verified end-to-end (PDF, HTML), written beside the source; collision naming and cancel paths checked
• Main app stays closed while an action runs
• Icons verified live in Login Items & Extensions — plates measure #7B7B81, identical to all six Apple built-ins (Rotate Left, Markup, Create PDF, Trim, Convert Image, Remove Background)
• Sanity-check exports on iCloud Drive and external volumes
• Verify appearance in dark mode

🤖 Generated with Claude Code