rename: skyai-finance → prism

Lifted from skyai-finance into a standalone theme once the choices
stabilised. New name earns itself twice: the literal glass refraction,
and the hue spectrum the corner glows span (lavender → peach → cyan → mint).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/validate-theme.yml

@@ -28,4 +28,4 @@ jobs:
           node .ci-scripts/check-theme.mjs \
             --target=theme.css \
             --reference=.ci-scripts/lib-theme-pristine/theme.css \
-            --name=skyai-finance
+            --name=prism