docs: add Vercel deployment path and clarify Supabase env errors

Prepare Gary for a simple one-person Vercel deployment. Documentation
and a clearer missing-env error - no product features, no Clerk, no
database migration, no Cloudflare changes.

- Add docs/VERCEL_DEPLOYMENT.md: plain-English guide covering what
  Vercel hosts (the Next.js frontend only), what Supabase handles,
  why the Express backend stays separate (needs LibreOffice and a
  long-running server), required public vs private environment
  variables, GitHub import steps, project settings (Root Directory =
  frontend, default build/output), preview deployments, custom
  domains, troubleshooting, and a "Future Clerk login" note.
- Add frontend/.env.vercel.example: public NEXT_PUBLIC_* values
  separated from server-side secrets, with explicit warnings never to
  put provider / Supabase service-role / database / JWT / Clerk
  secrets in NEXT_PUBLIC_* variables. Includes a clearly-labeled
  future-only Clerk section.
- supabase.ts and auth.ts: replace the cryptic "supabaseUrl is
  required" failure with a clear message naming the missing variable
  (NEXT_PUBLIC_SUPABASE_URL / NEXT_PUBLIC_SUPABASE_ANON_KEY) and
  where to set it. Also accept the standard
  NEXT_PUBLIC_SUPABASE_ANON_KEY name while still falling back to the
  legacy NEXT_PUBLIC_SUPABASE_PUBLISHABLE_DEFAULT_KEY, so existing
  deployments keep working. Runtime safety is unchanged - missing
  config still throws.
- README: add a brief "Deploying Gary on Vercel" section and link the
  new guide; docs index links it too.
- .gitignore: allow .env.vercel.example to be committed.