fix: support admin form creates

[vagdotdev]

Fixes a remaining create-route gap in the ky-backed API path: the admin page posts application/x-www-form-urlencoded data to /things/create, but the route only accepted JSON.

Changes:

• share the create request schema across JSON and URL-encoded form bodies
• keep JSON API creation working and assert the response in the existing ky test
• add a ky regression test for admin-style URL-encoded form creation
• return a 400 if neither supported body type is provided

Validation:

• bun test tests/routes/things/create.test.ts
• bun test
• bun run build
• bunx tsc --noEmit
• bun run format:check
• git diff --check

/claim #2