fix: resolve TypeScript compilation errors in Docker build

- Add vitest globals type definitions to tsconfig.app.json
- Enable globals in vite.config.ts test configuration
- Add explicit vitest imports and type references to all .test.ts files
- Add missing `placeholder` property to PasswordFieldProps interface
- Add missing `required` field to test field definitions in FieldRenderer.test.tsx
- Update FieldDef and BackendMeta interfaces to support readonly arrays
- Fix readonly array type incompatibilities in BackendSelectionStep.tsx
- Fix form submission handler type mismatch in RemoteConfigStep.tsx
- Remove unused helper functions and imports from StepIndicator.test.tsx

These changes resolve all TypeScript compilation errors preventing the Docker build from completing.

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

src/schemas/registry.ts

@@ -16,7 +16,7 @@ export interface FieldDef {
   required: boolean;
   placeholder?: string;
   helpText?: string;
-  options?: { value: string; label: string }[]; // for inputType: 'select'
+  options?: readonly { value: string; label: string }[]; // for inputType: 'select'
   validate?: { regex: RegExp; message: string };
   tooltipText?: string;
 }
@@ -25,7 +25,7 @@ export interface BackendMeta {
   displayName: string;
   description: string;
   category: BackendCategory;
-  fields: FieldDef[];
+  fields: readonly FieldDef[];
 }
 
 export const BACKEND_REGISTRY = {