fix: avoid Sanity 5 document-context crash in tool dialogs

[wdtamagi]

Problem

On Sanity 5, opening either of the plugin's tool dialogs (Add project or the Vercel Access Token form) crashes the Studio:

useDocumentDivergences must be used within a DocumentDivergencesContext

Root cause

Since Sanity 5, the core FormField exported from sanity renders FormDivergenceIndicator and FormNodeDivergenceDetail, and both call useDocumentDivergences() unconditionally. That hook throws when rendered outside a document pane (no DocumentDivergencesContext provider).

This plugin uses <FormField> inside its standalone tool dialogs (src/deploy-form.tsx and src/vercel-deploy.tsx), which are not inside a document pane — so the hook throws and the whole tool view errors out.

Fix

Add a small, context-free FormField shim (src/form-field.tsx) built from @sanity/ui (Stack + Text) that reproduces the same title/description layout without depending on document-editing context. Swap the two import { FormField } from 'sanity' usages to the local shim.

• src/form-field.tsx (new)
• src/deploy-form.tsx — import swap
• src/vercel-deploy.tsx — import swap (keeps useColorSchemeValue from sanity)

No other sanity imports are affected; they're all context-free.

Verification

• npm run build (pkg-utils build --strict --check) passes.
• Manually: on a Sanity 5 Studio, opening Add project and the access-token dialog now render correctly instead of crashing.