Give the win ending its own background image

Previously reused gameover.webp for both win and loss states since it
was deliberately mood-neutral. Now has a dedicated photo: same desk,
chair pushed back in rather than empty, stamp/ink/letters back in
place, a fresh unopened stack waiting — continuation, not triumph,
matching the ending copy's own restraint. .end-screen now carries a
victory/failure class the same way .end-card already did, so CSS can
target the right background per outcome.

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

src/style.css

@@ -81,7 +81,12 @@ button { font: inherit; }
     linear-gradient(rgba(13, 13, 26, 0.6), rgba(13, 13, 26, 0.88)),
     url('/intro.webp') center / cover no-repeat;
 }
-.end-screen {
+.end-screen.victory {
+  background:
+    linear-gradient(rgba(13, 13, 26, 0.55), rgba(13, 13, 26, 0.85)),
+    url('/win.webp') center / cover no-repeat;
+}
+.end-screen.failure {
   background:
     linear-gradient(rgba(13, 13, 26, 0.55), rgba(13, 13, 26, 0.85)),
     url('/gameover.webp') center / cover no-repeat;