Add a real intro background image

A generated desk/lamp/stamp photo (converted PNG -> WebP, 1.7MB ->
65KB, negligible quality loss at the size it's actually displayed)
now sits behind the intro screen, with the existing red "R" seal
stamped on top as an overlay — the seal already existed as a plain
CSS circle, just needed a photo worth stamping over. Gave it a subtle
dark backdrop + slightly stronger glow so it stays legible against
the busier background.

Raw source PNG gitignored — only the optimized derivative that
actually ships belongs in the repo.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -2,3 +2,7 @@ node_modules
 dist
 .env
 *.local
+
+# Raw source image — public/intro.webp is the optimized derivative that
+# actually ships; no need to bloat the repo with the 1.7MB original.
+/intro.png