style: update various code copy button and toc styles

- Removed the old copy-action SVG icon and replaced it with a new copy-button SVG icon.
- Added a new copy-success SVG icon for visual feedback on copy actions.
- Updated Waline component comments for clarity and improved code readability.
- Enhanced BackButton component with fallback navigation to homepage.
- Improved CodeCopyButton component by refining icon handling and timeout management.
- Adjusted GithubCard component's intersection observer root margin for better visibility.
- Updated GsapAnimation component variable names for consistency and clarity.
- Removed unnecessary styles from PhotoSwipe component.
- Modified TOC component styles for better spacing and layout.
- Updated multiple theme guide markdown files to correct image hosting URL comments.
- Cleaned up extend.css by removing commented-out styles and organizing code.
- Added PhotoSwipe background color and KaTeX overflow fixes to global.css.
- Enhanced markdown.css with video styling for better responsiveness.
- Improved transition.css comments for better understanding of GSAP animation states.

src/assets/icons/copy-check.svg

@@ -1,3 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24">
-  <path d="M9 20 .9 11.9l1.2-.6L9 17.6 22.5 5l.6 1z"/>
-</svg>