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/styles/markdown.css

@@ -75,6 +75,17 @@
   --at-apply: 'mb-4';
 }
 
+/* Video */
+.heti iframe {
+  --at-apply: 'my-6 aspect-video w-full';
+}
+.heti :is(h1, h2, h3, h4, h5, h6, iframe) + iframe {
+  --at-apply: 'mt-4';
+}
+.heti iframe:has(+ iframe) {
+  --at-apply: 'mb-4';
+}
+
 /* Code Blocks */
 .heti :where(pre) {
   --at-apply: 'overflow-auto uno-round-border px-4 py-3 bg-secondary/5!';