Add Vercel preview deployments for faster PR feedback

### 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:** #2147 — Vercel account must be created and repo linked before these config changes take effect
- **Parent epic:** #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](https://vercel.com/docs/deployments/preview-deployments)
- [Vercel Rewrites / Proxy configuration](https://vercel.com/docs/projects/project-configuration#routes)
- Branch with implementation: `feat/vercel-deployment`