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>

.gitignore

@@ -7,3 +7,5 @@ dist
 # actually ship; no need to bloat the repo with the 1.7MB originals.
 /intro.png
 /gameover.png
+/win.png
+/win2.png