Redesign UI with modern dark theme and improved layout

[CrazyWolf13]

Purpose

Based on user feedback about the background looking weird and the interface appearing unchanged, this PR implements a complete visual redesign to address UI/UX concerns and modernize the application's appearance.

Code changes

• Complete UI overhaul: Replaced basic styling with modern dark theme using CSS custom properties and gradient backgrounds
• Typography upgrade: Added Inter font family for improved readability and modern appearance
• Layout restructuring: Introduced hero header section with title and subtitle, wrapped main content in app shell container
• Component styling improvements: Enhanced StreamlinkUi with better spacing, card-based layouts, and refined button styles
• StyledButton refactor: Converted from gradient prop to variant-based system with improved hover effects
• Package management: Added yarn package manager specification and fixed Vue app mounting
• Responsive design: Improved grid layouts and spacing for better mobile/desktop experienceTo clone this PR locally use the Github CLI with command gh pr checkout 7

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