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/pages/home.html

@@ -5,17 +5,20 @@
   <div class="hero-copy">
     {{if .CurrentUser}}
     <p class="hero-eyebrow">Welcome back, {{.CurrentUser.Username}}</p>
-    <h1 id="upload-title">Drop it. Share it.</h1>
-    <p>{{.Data.LimitSummary}}</p>
     {{else}}
     <p class="hero-eyebrow">Welcome</p>
-    <h1 id="upload-title">Send a file. Get a link.</h1>
-    <p>Fast, private transfers that expire on your terms.</p>
     {{end}}
   </div>
 
   <form class="upload-panel card" id="upload-form" action="/api/v1/upload" method="post" enctype="multipart/form-data">
     <div class="card-content">
+      {{if .CurrentUser}}
+      <h1 id="upload-title">Drop it. Share it.</h1>
+      <p class="upload-subtitle">{{.Data.LimitSummary}}</p>
+      {{else}}
+      <h1 id="upload-title">Send a file. Get a link.</h1>
+      <p class="upload-subtitle">Fast, private transfers that expire on your terms.</p>
+      {{end}}
       <label class="drop-zone" for="file-input">
         <span class="drop-icon" aria-hidden="true">
           <svg viewBox="0 0 24 24" role="img" focusable="false" aria-hidden="true"><path d="M12 16V4m0 0 4 4m-4-4-4 4M5 20h14" /></svg>