style: update popup width and document styles

- Change popup width from 800px to 71rem for better scaling
- Add DM Sans as default body font
- Set h1 font-size to 3rem
- Reduce paragraph bottom margin from 10mm to 5mm
- Add link styles (purple color, no underline)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

public/assets/css/src/_settings-popup.scss

@@ -6,7 +6,7 @@
   border-radius: 8px;
   box-shadow: 0 4px 20px rgba(0, 0, 0, 0.2);
   z-index: 10000;
-  width: 800px;
+  width: 71rem;
   max-height: 600px;
   display: flex;
   flex-direction: column;