Update Next.js to version 15.4.2 and refactor invoice pages

- Upgrade Next.js and related packages for improved performance and security
- Refactor invoice-related pages to streamline navigation and enhance user experience
- Consolidate invoice editing and viewing functionality into a single page
- Remove deprecated edit page and implement a new view page for invoices
- Update links and routing for consistency across the dashboard

env.example

@@ -0,0 +1,24 @@
+# Base application env (example)
+NODE_ENV=production
+PORT=3000
+HOSTNAME=0.0.0.0
+
+# NextAuth
+AUTH_SECRET=__GENERATE__
+
+# Database (Postgres)
+POSTGRES_USER=beenvoice
+POSTGRES_PASSWORD=__GENERATE__
+POSTGRES_DB=beenvoice
+DATABASE_URL=postgres://${POSTGRES_USER}:${POSTGRES_PASSWORD}@db:5432/${POSTGRES_DB}
+# Disable SSL for Docker local Postgres; set to false or remove for managed Postgres
+DB_DISABLE_SSL=true
+
+# Email (Resend). Replace with real keys in production
+RESEND_API_KEY=replace-or-remove
+RESEND_DOMAIN=
+
+# Build tweaks
+SKIP_ENV_VALIDATION=1
+
+