## Problem
Project creation can send a request without a region, which the API
rejects. Empty region values pass frontend validation, and a failed
available-regions request fills in a static region name that cannot
resolve to a region selection.
## Fix
Keep the region selector controlled from mount and require a nonempty
value. Disable creation when available regions fail to load, guard
submission against unresolved region selections, and preserve the static
fallback for providers that use it.
Fixes
[FE-3884](https://linear.app/supabase/issue/FE-3884/project-creation-wizard-can-submit-with-an-empty-region).
## How to test
- Open the new-project form and verify the recommended region submits
without manually selecting a region.
- Submit with an empty region. Expected result: an inline validation
error appears and no project creation request is sent.
- Fail the available-regions endpoint, including after changing compute
size with a region already selected. Expected result: the region-loading
error appears, creation is disabled, and direct form submission sends no
project creation request.
- Run `pnpm --filter studio exec vitest --run
'tests/pages/new/[slug].test.tsx'
tests/pages/vercel-deploy-button-new-project.test.tsx
components/interfaces/ProjectCreation/ProjectCreation.schema.test.ts`.
Local validation: all 54 focused tests and formatting pass. Focused
ESLint has no errors. Local typecheck reports the existing image type
mismatch in InstructionBlocks.tsx; CI typecheck passed for the previous
commit.