feat(theme): introduce retro Windows 98-inspired theme

Add a new "retro" theme option that mimics the classic Windows 98 aesthetic, providing a nostalgic alternative to the modern and classic dark themes.

Changes include:
- Defining CSS variables for the retro theme in `00-base.css` (pixel fonts, silver/gray colors, and classic window shadows).
- Adding custom styling for cards, headers, buttons, and title bars to replicate classic OS windows.
- Adding a star background GIF (`stars1.gif`).
- Excluding the retro theme from modern "revamp" styles in `15-revamp.css`.
- Updating `CLAUDE.md` with instructions on screenshot verification.

backend/static/js/05-theme.js

@@ -11,7 +11,7 @@
  */
 (function () {
   var STORAGE_KEY = "warpbox-theme";
-  var THEMES = ["revamp", "classic"];
+  var THEMES = ["revamp", "classic", "retro"];
 
   function stored() {
     try {