Fix markdown preview wrapping and Windows open-with registration

[KarlDavidRosen]

Summary

• Left-align markdown preview content instead of centering it.
• Improve wrapping for paragraphs, lists, inline code, code blocks, and tables so content stays visible in the window.
• Register "Open with MarkPad" for .md and .markdown files during Windows NSIS installation.
• Remove those registry entries on uninstall.

Verification

• npm run check
• npm run build
• npm run tauri -- build --target x86_64-pc-windows-msvc --bundles nsis --ci

Notes:

• Existing Svelte accessibility warnings remain unchanged.
• Windows NSIS build completed successfully.