Add new built-in themes and update theme selection UI

Introduces seven new built-in themes (cyberpunk, midnight-rose, arctic-ice, cherry-blossom, cosmic-purple, emerald-dream, mocha-coffee, lavender-fields) with corresponding JSON files, updates the FEATURES documentation, and enhances the settings UI to display and select these themes. Also updates theme loading logic in theme-manager.js and improves the theme selector layout for better usability.

themes/cosmic-purple.json

@@ -0,0 +1,17 @@
+{
+  "name": "Cosmic Purple",
+  "colors": {
+    "bg": "#0f0524",
+    "darkBlue": "#1a0b3d",
+    "darkPurple": "#2d1b69",
+    "primary": "#8a2be2",
+    "accent": "#da70d6",
+    "text": "#e6e6fa"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #0f0524 0%, #2d1b69 50%, #4b0082 100%)",
+  "version": "1.0",
+  "description": "A space-inspired theme with deep cosmic purples and starlight accents"
+}