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/css/20-upload.css

@@ -29,6 +29,13 @@
 }
 
 
+.upload-subtitle {
+  margin: 0.35rem 0 1.25rem;
+  color: var(--muted-foreground);
+  font-size: 0.95rem;
+  line-height: 1.5;
+}
+
 .drop-zone {
   min-height: 19rem;
   display: grid;