DRY up ZodError → FieldError mapping

Five journal API route handlers had the same inline lambda:

    parsed.error.issues.map((i) => ({
      field: i.path.join("."),
      message: i.message,
    }))

Pull it into `zodIssuesToFieldErrors(error)` next to FieldError in
`@trails-cool/api/errors.ts` and re-export it so all five handlers
share the same implementation. The path-flattening rule now lives in
one place, and new routes get it for free.

Public error-response shape is unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/api/src/index.ts

@@ -4,6 +4,7 @@ export { ENDPOINTS } from "./endpoints.ts";
 // Error schemas
 export {
   ApiErrorResponseSchema, FieldErrorSchema, ERROR_CODES,
+  zodIssuesToFieldErrors,
   type ApiErrorResponse, type FieldError,
 } from "./errors.ts";