Add Vercel preview deployments for faster PR feedback

[trillium]

Overview

Enable Vercel preview deployments so that maintainers and contributors can see their commits deployed automatically on each push/PR. Preview deployments proxy API requests to the existing dev backend, giving reviewers a live, functional preview rather than requiring a local setup to verify frontend changes.

Dependencies

• Blocked by: Create Vercel account under Hack for LA org Gmail and link VRMS repo #2147 — Vercel account must be created and repo linked before these config changes take effect
• Parent epic: Epic: Vercel preview deployments for frontend PRs #2146

Action Items

• Add .vercel and .env*.local to root and client .gitignore files to keep deployment credentials out of version control
• Update the client build script to handle CI environments (disable warnings-as-errors, OpenSSL legacy provider compatibility)
• Add client/vercel.json to configure build output, API proxy routing to dev.vrms.io, and SPA fallback for client-side routing
• Connect the repo to Vercel so preview deployments are triggered on PRs targeting development

Resources/Instructions

• Vercel Preview Deployments docs
• Vercel Rewrites / Proxy configuration
• Branch with implementation: feat/vercel-deployment

[trillium]

Prerequisite

• Create a Vercel account under the Hack for LA Gmail and link it to the hackforla/VRMS repo
• Ensure the Vercel project is configured to deploy from the client/ directory
• Set the framework preset to Create React App

This must be done before preview deployments will work on PRs.