Production webapp deploy must be manual (workflow_dispatch only) #2934

Problem

.github/workflows/vercel-prod.yaml deploys the production webapp on every push to main touching crates/**, packages/** (and deps/flake/build script). Front-end code merged to main therefore ships to prod before the contracts it calls are deployed — e.g. the V7 change (#2929) would go live while settings still point at the V6 raindex.

Ruling

"webapp on dex to be manual": the production webapp deploy runs only via workflow_dispatch, with no push trigger. PR/branch preview deploys are unchanged.

Out of scope

vercel-docs-prod.yaml (typedoc for packages/raindex, separate Vercel docs project) still deploys on every main push; it is not the webapp and is not changed here.