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/templates/layouts/base.html

@@ -19,6 +19,7 @@
   <link rel="stylesheet" href="/static/css/00-base.css">
   <link rel="stylesheet" href="/static/css/10-layout.css">
   <link rel="stylesheet" href="/static/css/15-revamp.css">
+  <link rel="stylesheet" href="/static/css/16-retro.css">
   <link rel="stylesheet" href="/static/css/20-upload.css">
   <link rel="stylesheet" href="/static/css/30-download.css">
   <link rel="stylesheet" href="/static/css/40-docs.css">
@@ -65,6 +66,7 @@
       <select data-theme-select aria-label="Site theme">
         <option value="revamp">Aurora (default)</option>
         <option value="classic">Classic</option>
+        <option value="retro">Web 1.0 (retro)</option>
       </select>
     </label>
     <span class="footer-links">{{if .CurrentUser}}<a href="/app">Dashboard</a><a href="/api">API</a><a href="/account/settings">Account</a>{{else}}<a href="/login">Sign in</a><a href="/api">API</a>{{end}}</span>