Stop tracking banner.json runtime data store

Same reasoning as scheduled-emails.json: it's mutated by the running
app (site banner content), not source. readBanner() already falls
back to sane defaults when the file is missing, so untracking it is
safe for fresh checkouts.

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

.gitignore

@@ -36,4 +36,5 @@ temp/
 backend/public/uploads/
 
 # runtime data stores (mutated by the running app, not source)
-backend/data/scheduled-emails.json
+backend/data/scheduled-emails.json
+backend/data/banner.json

backend/data/banner.json

@@ -1,6 +0,0 @@
-{
-  "message": "",
-  "type": "info",
-  "liveFrom": null,
-  "liveTill": null
-}