fix: resolve remaining TypeScript compilation errors

- Add placeholder property to TextFieldMD3 component and props interface
- Fix vite.config.ts to import defineConfig from vitest/config instead of vite
- Add type guard for validate property in schemas/index.ts to handle readonly array union types
- Remove unused StepIndicatorWithDispatch placeholder function from tests

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/schemas/index.ts

@@ -15,7 +15,7 @@ function buildZodSchema(backendType: BackendType): z.ZodObject<Record<string, z.
       ? z.string().min(1, `${field.label} is required`)
       : z.string();
 
-    if (field.validate) {
+    if ('validate' in field && field.validate) {
       schema = (schema as z.ZodString).regex(field.validate.regex, field.validate.message);
     }