ci: Deploy only on push to master

The deploy workflow used `on: push` with `branches: []`, which GitHub
treats as no filter — so it ran on pushes to every branch (including PR
branches). Restrict it to the master branch so it deploys only on merge.

Author: TamimEhsan

.github/workflows/nextjs.yml

@@ -5,9 +5,9 @@
 name: Deploy Next.js site to Pages
 
 on:
-  # Runs on pushes targeting the default branch
+  # Runs on pushes targeting the default branch (i.e. on merge to master)
   push:
-    branches: []
+    branches: ["master"]
 
   # Allows you to run this workflow manually from the Actions tab
   workflow_dispatch: