feat: Add Markdown rendering support with various elements and parsin…

[2md55nmxkw-arch]

Summary

This PR significantly expands WindUI with rich content rendering, responsive sidebar behavior, persistent themes and plugins, profile improvements, and a complete feature showcase.

Added

• Native Markdown renderer with:

  • headings, quotes, dividers, code blocks;
  • tables with alignment;
  • images and video blocks;
  • nested lists and task lists;
  • runtime content updates via SetContent().

• HTML-lite renderer that converts HTML content into WindUI-native Markdown blocks.

• New UI elements:

  • HTML
  • Markdown
  • Image
  • Video
  • ProgressBar

• Background image support for element surfaces through:

  • BackgroundOverlayTransparency
  • BackgroundElementTransparency

• Responsive mobile sidebar:

  • animated drawer;
  • mobile menu button before the title;
  • safe clipping inside the window;
  • configurable width, padding, and gap;
  • desktop sidebar collapse support.

• Sidebar API:

  • Window:SetSidebarOpen(boolean)
  • Window:ToggleSidebar()
  • Window:SetMobileNavigation(boolean)

• Theme API:

  • CreateTheme
  • EditTheme
  • RemoveTheme
  • SaveTheme
  • LoadTheme
  • ListSavedThemes
  • DeleteSavedTheme

• Plugin system:

  • plugin lifecycle with Init and Destroy;
  • enable/disable support;
  • persistent plugin state;
  • disk manifest loader from WindUI/plugins/*.lua;
  • plugin manager table with name, version, and status.

• Profile improvements:

  • profile listing;
  • create, select, save, load, rename, and delete;
  • autoload support.