feat(05-01): wire BackendSelectionStep to registry and remove dead export

- Replace hardcoded BACKENDS array with Object.entries(BACKEND_REGISTRY)
- Import BACKEND_REGISTRY in BackendSelectionStep.tsx
- Remove BackendFormValues<T> export from schemas/index.ts (dead code TECH-04)
- TypeScript compiles with zero errors

src/schemas/index.ts

@@ -23,6 +23,3 @@ export const BACKEND_SCHEMAS = {
   's3-compatible':  buildZodSchema('s3-compatible'),
 } as const;
 
-// Utility type: infer TypeScript type from a backend's Zod schema
-export type BackendFormValues<T extends BackendType> =
-  z.infer<typeof BACKEND_SCHEMAS[T]>;