feat(export): add theme selection to PDF, HTML & Image exports and fix dark/light styling

[itznan]

Summary

This PR enhances export capabilities and fixes several styling and parsing edge cases across exports:

1. Theme Selection Dialogs for Exports

• Export PDF: Added dedicated Light Theme and Dark Theme selectable options in the PDF export modal.
• Export HTML: Added dedicated Light Theme and Dark Theme selectable options in the HTML export modal.
• Export PNG: Added dedicated Light Theme and Dark Theme selectable options in the Image export modal.

2. Export & Preview Rendering Fixes

• Dark Mode PDF Export:
  • Fixed white borders in raster PDF export by pre-filling page backgrounds in jsPDF and canvas render contexts with #0d1117.
  • Fixed vector PDF (window.print) @page margins and edge-to-edge dark background coverage.
  • Enhanced GitHub Alerts (> [!NOTE], > [!TIP], etc.) with dark backgrounds and color accents.
  • Replaced double-line bevel borders on --- horizontal dividers with clean dark styling.
• Light Mode HTML Export:
  • Fixed tables appearing with dark backgrounds when viewed on dark-mode OS environments by explicitly setting data-color-mode="light" and overriding canvas variables.
  • Fixed Mermaid diagrams showing "Rendering Mermaid…" text in exported HTML by stripping loading status elements and properly initializing and invoking mermaid.run().
• Frontmatter Parsing Hardening:
  • Prevented documents containing multiple --- dividers from incorrectly being treated as YAML frontmatter.

3. Desktop Offline Assets

• Synchronized all changes to offline desktop resources via prepare.js.

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Project	Deployment	Actions	Updated (UTC)
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[itznan]

Hi @ThisIs-Developer,

Here is a summary of the updates and fixes implemented in this PR:

🎯 Key Changes & Enhancements

1. Export Theme Selection Dialogs:

   • Added selectable Light / Dark theme options to the PDF, HTML, and Image (PNG/JPEG/WebP) export modals.
   • Users can now export in their preferred theme regardless of the current app viewing mode.

2. Dark Mode PDF Export Overhaul:

   • Raster PDF (jsPDF + html2canvas): Fixed white border/margin seams by pre-filling page contexts with #0d1117 in jsPDF and setting canvas background color to #0d1117.
   • Browser Print (Vector PDF): Injected data-browser-print-export="dark" with dedicated @media print CSS rules covering code blocks (#161b22), tables, horizontal rules, syntax tokens (--color-prettylights-*), and GitHub Alerts ([!NOTE], [!TIP], [!WARNING], [!CAUTION], [!IMPORTANT]).

3. Standalone HTML Export Improvements:

   • System Theme Isolation: Explicitly enforced data-color-mode and canvas variables so Light HTML exports do not inherit unwanted dark backgrounds from OS dark mode preferences.
   • Diagram Rendering: Removed loading placeholders (.diagram-status, .diagram-toolbar) and correctly initialized/executed mermaid.run() with the target theme so Mermaid diagrams render cleanly in exported HTML.

4. Frontmatter Parsing Hardening:

   • Tightened regex checks in preview-worker.js and script.js to ensure horizontal rule dividers (---) later in the document do not trigger false YAML frontmatter blocks.

5. Desktop Sync & E2E Test Coverage:

   • Synchronized all web updates to offline desktop assets via prepare.js.
   • Added automated E2E test coverage for export theme options in tests/e2e/export.spec.js.

The branch has also been rebased and synced with the latest main. Looking forward to your review!

[ThisIs-Developer]

Hi @itznan, I really like the direction of this PR—especially the theme-aware export options and rendering improvements.

I have one UI suggestion:

1. Please make the theme selector more compact and modern. Also, remember the last selected theme for each export type, so reopening the dialog restores the user’s previous selection instead of resetting it to the current application theme.

Thank you for the work!

[itznan]

Hi @ThisIs-Developer,

I've addressed your UI feedback in commit a0a86c0:

1. Compact & Modern Theme Selector:

   • Redesigned the theme selection across PDF, HTML, and Image export modals using a sleek 2-option segmented toggle (.export-theme-toggle) with light/dark icons.
   • Removed the bulky card layouts and descriptions to keep the dialogs compact, clean, and consistent with the app's design system.

2. Persistent Theme Selection per Export Type:

   • Saved and restored user theme choices independently for PDF, HTML, and Image exports (\markdownViewerExportPdfTheme, \markdownViewerExportHtmlTheme, \markdownViewerExportPngTheme).
   • Reopening any export dialog now preserves the user's last selected export theme instead of resetting to the active app viewing theme.
   • Synchronized preferences with desktop Neutralino storage.

3. Desktop Sync & Tests:

   • Synchronized all changes to offline desktop resources via \prepare.js.
   • Added automated E2E test coverage in \ ests/e2e/export.spec.js\ verifying the compact toggle layout and theme memory across dialog opens.

Ready for your review!

[ThisIs-Developer]

Thank you @itznan for implementing the requested changes.

The updated theme selector and remembered export preferences are a great addition and make the export experience much better.

Thank you again for the contribution and for improving Markdown Viewer!