fix(ci): add BETTER_AUTH_SECRET for projects example build

Copy .env.example to .env during CI build to provide required
BETTER_AUTH_SECRET that better-auth now enforces.

Author: claude

.github/workflows/pr.yml

@@ -89,4 +89,5 @@ jobs:
       - name: Build Starter Site
         run: |
           cd examples/react/projects
+          cp .env.example .env
           pnpm build

examples/react/projects/.env.example

@@ -3,4 +3,4 @@
 DATABASE_URL=postgresql://postgres:password@localhost:54321/projects
 
 # Create a secret for better-auth
-BETTER_AUTH_SECRET=
+BETTER_AUTH_SECRET=example-secret-for-development-only