chore: add Firebase Hosting config for season content delivery

firebase init hosting wired to project block-seasons, public dir = deploy.
Generated season payload (deploy/content/) and CLI cache (.firebase/) are
gitignored — content/ is the source of truth, regenerated by the deploy
script in docs/firebase-hosting-guide.md.

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

.gitignore

@@ -49,3 +49,7 @@ lib/l10n/gen/
 .superpowers/
 CLAUDE.md
 *.pid
+
+# Firebase Hosting (CLI deploy cache + generated content payload)
+.firebase/
+/deploy/content/