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/cherry-blossom.json

@@ -0,0 +1,17 @@
+{
+  "name": "Cherry Blossom",
+  "colors": {
+    "bg": "#fff5f8",
+    "darkBlue": "#ffe4e8",
+    "darkPurple": "#ffd4db",
+    "primary": "#ff69b4",
+    "accent": "#ffb6c1",
+    "text": "#8b4513"
+  },
+  "layout": "centered",
+  "showLogo": true,
+  "customTitle": "Nebula Browser",
+  "gradient": "linear-gradient(145deg, #fff5f8 0%, #ffd4db 100%)",
+  "version": "1.0",
+  "description": "A soft, warm theme inspired by cherry blossoms with gentle pink tones"
+}