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/lavender-fields.json

@@ -0,0 +1,17 @@
+{
+  "name": "Lavender Fields",
+  "colors": {
+    "bg": "#f8f4ff",
+    "darkBlue": "#ede4ff",
+    "darkPurple": "#e6d8ff",
+    "primary": "#9370db",
+    "accent": "#dda0dd",
+    "text": "#4b0082"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #f8f4ff 0%, #e6d8ff 100%)",
+  "version": "1.0",
+  "description": "A peaceful light theme inspired by lavender fields with soft purple hues"
+}