feat(03-03): implement BackendSelectionStep (WIZD-01, WIZD-04)

- Remote name field with zod validation (mode: onSubmit, reValidateMode: onChange)
- Three backend cards in popularity order: Azure Blob, Amazon S3, S3-Compatible
- Card click validates name first; dispatches SET_REMOTE_NAME + SET_BACKEND_TYPE + SET_REMOTE_PARAMS({}) + SET_STEP(1)
- Does NOT dispatch RESET — preserves deployment options
- All 10 BackendSelectionStep tests GREEN

src/components/wizard/BackendSelectionStep.test.tsx

@@ -2,7 +2,7 @@
 // Covers WIZD-01: card grid renders Azure Blob, Amazon S3, S3-Compatible (Azure first)
 // Covers WIZD-04: remote name field validates alphanumeric/dash/underscore
 import { describe, it, expect } from 'vitest';
-import { render, screen, fireEvent } from '@testing-library/react';
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
 import { WizardProvider } from '../../store/context';
 import { BackendSelectionStep } from './BackendSelectionStep';
 
@@ -76,35 +76,41 @@ describe('BackendSelectionStep', () => {
       expect(screen.queryByRole('alert')).toBeNull();
     });
 
-    it('shows inline error after first Next attempt with invalid name', () => {
+    it('shows inline error after first Next attempt with invalid name', async () => {
       renderStep();
       // Click a card without filling in name — triggers validation
       const azureButton = screen.getAllByRole('button').find(b => b.textContent?.includes('Azure Blob Storage'))!;
       fireEvent.click(azureButton);
-      // Error should appear
+      // Error should appear after async validation
+      await waitFor(() => {
         expect(screen.getByRole('alert')).toBeDefined();
         expect(screen.getByText(/required/i)).toBeDefined();
       });
+    });
 
-    it('accepts alphanumeric, dashes, and underscores', () => {
+    it('accepts alphanumeric, dashes, and underscores', async () => {
       renderStep();
       const nameInput = screen.getByRole('textbox');
       fireEvent.change(nameInput, { target: { value: 'my-remote_01' } });
       const azureButton = screen.getAllByRole('button').find(b => b.textContent?.includes('Azure Blob Storage'))!;
       fireEvent.click(azureButton);
-      // Valid name — no alert should appear
+      // Valid name — no alert should appear (wait briefly to confirm no error appears)
+      await waitFor(() => {
         expect(screen.queryByRole('alert')).toBeNull();
       });
+    });
 
-    it('rejects names with spaces or special characters', () => {
+    it('rejects names with spaces or special characters', async () => {
       renderStep();
       const nameInput = screen.getByRole('textbox');
       fireEvent.change(nameInput, { target: { value: 'my remote!' } });
       const azureButton = screen.getAllByRole('button').find(b => b.textContent?.includes('Azure Blob Storage'))!;
       fireEvent.click(azureButton);
-      // Invalid name — alert should appear
+      // Invalid name — alert should appear after async validation
+      await waitFor(() => {
         expect(screen.getByRole('alert')).toBeDefined();
         expect(screen.getByText(/letters, numbers, dashes/i)).toBeDefined();
       });
     });
+  });
 });

src/components/wizard/BackendSelectionStep.tsx

@@ -0,0 +1,93 @@
+import { useRef } from 'react';
+import { useForm } from 'react-hook-form';
+import { zodResolver } from '@hookform/resolvers/zod';
+import { z } from 'zod';
+import { useWizard } from '../../store/context';
+import type { BackendType } from '../../store/types';
+import { BackendCard } from '../ui/BackendCard';
+
+const remoteNameSchema = z.object({
+  name: z
+    .string()
+    .min(1, 'Remote name is required')
+    .regex(
+      /^[a-zA-Z0-9_-]+$/,
+      'Only letters, numbers, dashes, and underscores allowed'
+    ),
+});
+
+type RemoteNameFormValues = z.infer<typeof remoteNameSchema>;
+
+const BACKENDS: { type: BackendType; name: string; description: string }[] = [
+  {
+    type: 'azureblob',
+    name: 'Azure Blob Storage',
+    description: 'Microsoft Azure cloud storage',
+  },
+  {
+    type: 's3',
+    name: 'Amazon S3',
+    description: 'AWS Simple Storage Service',
+  },
+  {
+    type: 's3-compatible',
+    name: 'S3-Compatible',
+    description: 'Wasabi, MinIO, Cloudflare R2, and others',
+  },
+];
+
+export function BackendSelectionStep() {
+  const { state, dispatch } = useWizard();
+  const pendingBackend = useRef<BackendType | null>(null);
+
+  const {
+    register,
+    handleSubmit,
+    formState: { errors },
+  } = useForm<RemoteNameFormValues>({
+    resolver: zodResolver(remoteNameSchema),
+    mode: 'onSubmit',
+    reValidateMode: 'onChange',
+    defaultValues: { name: state.remote.name },
+  });
+
+  function onValidSubmit(values: RemoteNameFormValues) {
+    const backend = pendingBackend.current;
+    if (!backend) return;
+    dispatch({ type: 'SET_REMOTE_NAME', payload: values.name });
+    dispatch({ type: 'SET_BACKEND_TYPE', payload: backend });
+    dispatch({ type: 'SET_REMOTE_PARAMS', payload: {} });
+    dispatch({ type: 'SET_STEP', payload: 1 });
+  }
+
+  function handleCardClick(backendType: BackendType) {
+    pendingBackend.current = backendType;
+    void handleSubmit(onValidSubmit)();
+  }
+
+  return (
+    <div>
+      <h2>Step 1: Select Backend</h2>
+      <form onSubmit={handleSubmit(onValidSubmit)}>
+        <div>
+          <label htmlFor="remote-name">Remote name</label>
+          <input id="remote-name" type="text" {...register('name')} />
+          {errors.name && (
+            <p role="alert">{errors.name.message}</p>
+          )}
+        </div>
+        <div data-testid="backend-cards">
+          {BACKENDS.map((backend) => (
+            <BackendCard
+              key={backend.type}
+              name={backend.name}
+              description={backend.description}
+              selected={state.remote.backendType === backend.type}
+              onClick={() => handleCardClick(backend.type)}
+            />
+          ))}
+        </div>
+      </form>
+    </div>
+  );
+}