[Port]: PreviewMD on Windows #22

Target platform

Windows 11 (works on Windows 10 22H2+ too)

Proposed native stack and embedded web view

WinUI 3 (C#/.NET 8) + WebView2

First vertical slice

One WinUI 3 window that opens a local Markdown file and renders it fully offline. It reuses the renderer from Sources/PreviewMD/Resources/Renderer/ unchanged.

1. Offline rendering. The build script copies the Renderer folder into the MSIX package and verifies it by checksum. WebView2 loads it through SetVirtualHostNameToFolderMapping. markdown-it, Mermaid, KaTeX and highlight.js all come from the package, nothing from the network.
2. Open a local file. A FileOpenPicker filtered to .md/.markdown. The shell reads the file and passes the content plus its folder path to the renderer, following the "load Markdown" contract from docs/PORTING.md. With no file open, the window shows an empty state with an Open button.
3. Relative images. The shell serves files only from the opened document's folder, so ![alt](img/pic.png) resolves offline and nothing outside that folder is readable.
4. One bridge action. Clicking an external link opens it in the default browser via Launcher.LaunchUriAsync. Local links stay in the document.

Done means: open a .md file, it renders offline with diagrams, math and code highlighting, a relative image shows up, an external link opens in the browser.

How will this feel native to the platform?

• WinUI 3 window with the standard Windows title bar, snap layouts, Ctrl+O/Ctrl+W/Ctrl+Tab shortcuts and native context menus. Tabs in a TabView come after the slice works.
• Follows the Windows dark/light theme.
• Native file dialogs, drag and drop of .md files onto the window, recent files in the Jump List.
• MSIX package. Per docs/PORTING.md I won't copy the macOS toolbar. Windows gets Windows patterns.

Files, sandboxing, links, and updates

• The renderer runs in WebView2's sandboxed Chromium processes. The only privileged part is the C# bridge with three operations: load document, resolve resource, open link.
• MSIX packages are read-only, so the bundled renderer assets can't change at runtime. The build pins them by checksum. The app makes no network requests.
• File access goes through the picker, so the user chooses what to open. The resource resolver only touches the opened document's folder. External links always go out to the default browser and never render inside the app.
• Updates: dev-signed MSIX for the prototype. Store or sideload distribution only if the port matures, and per TRADEMARKS.md.

Maintenance plan

I'm @kamilkaczmareksolutions, working on Windows, no Mac involved. This starts as a proof of concept: I'll build the slice and iterate through review. If the project develops well, I'm open to staying on as the Windows maintainer. If not, I'll leave the experiment documented and easy for someone else to pick up.

Before submitting

• I read docs/PORTING.md and will begin with a thin experiment rather than rewriting the macOS app.
• I understand that public use of the PreviewMD name and icon follows TRADEMARKS.md.