Web: Configure Vercel and document environment variables

[pvgstudio-design]

Configure apps/web to deploy on Vercel and document environment variables and domain setup.

Tasks:

• Create a Vercel project for the apps/web directory.
• Add staging and production environments and document where to set env vars.
• Add deployment instructions and test that the staging site calls the staging API endpoint.

Acceptance criteria:

• apps/web deploys automatically on push to main.
• Staging site calls the staging API endpoint successfully.

Estimate: 0.5–1 day