Add Share as PDF and Share Note File to share menu

[gjouret]

Summary

• Adds Share as PDF option to the share menu: renders the note to HTML, converts to a multi-page A4 PDF via WKWebView.createPDF(), and opens the system share sheet with the PDF file
• Adds Share Note File option: shares the note's file/directory directly via the system share sheet (works for both plain .md files and .textbundle packages)
• New PDFExporter helper class (follows the same WKWebView + WKNavigationDelegate pattern as the existing Printer.swift)
• Fixes PDF export missing images: buildPage() previously skipped loadImages() when print: true, relying on assignBase64Images() which only matched standard ![alt](path) syntax — images inside markdown tables were missed. Removed the !print guard so loadImages() copies image files to the temp bundle for all export modes.

Implementation Details

• PDF rendering uses MPreviewView.buildPage() to generate HTML, then loads it in a headless WKWebView
• After content loads, JavaScript measures document.body.scrollHeight and the web view is resized to the full content height before calling createPDF() — this ensures multi-page documents render completely
• The PDFExporter is retained via a static property on EditorViewController to prevent deallocation during async rendering
• Both new options appear in the existing NSSharingServicePicker alongside Web, Copy Plain Text, and Copy HTML
• loadImages() now runs for print/PDF export mode too — it copies image files from the note's assets/ directory to the temp bundle so the headless WKWebView can resolve relative <img src> paths
• Requires macOS 11.0+ (same as existing share menu items)

Test plan

• Select a note and click the share button — verify "Share as PDF" and "Share Note File" appear in the menu
• Share as PDF on a short note — verify a single-page PDF is created and the share sheet opens
• Share as PDF on a long note with multiple pages — verify all pages are included in the PDF
• Share as PDF on a note with inline images — verify images appear in the exported PDF
• Share as PDF on a note with a PDF thumbnail table — verify the thumbnail renders in the PDF
• Share Note File on a .md note — verify the file URL is shared
• Share Note File on a .textbundle note — verify the bundle directory is shared
• Test from both the main window and a note opened in a separate window

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