feat(intake): offer background removal on the submission page, ticked (#281)

Adds a checkbox to the public submission page that lets a sender opt out of background removal, ticked by default because most items look better cut out and the reverse default would mean almost nobody got it. It only renders when the server reports the sidecar is configured, matching the intake link's new backgroundRemoval flag from Task 5 — an unconfigured environment gets no checkbox rather than one that would do nothing.

submitItem now takes removeBackground as a required fourth parameter, sent as the multipart string 'true' or 'false' to match the backend's exact-string opt-out contract. Making the parameter required rather than optional was deliberate, so the compiler would catch any call site left unupdated; the frontend build (which also type-checks tests/ via tsconfig.test.json) confirmed the only call site, in Submit.tsx, was updated.

scripts/start-local.ps1 now sets REMBG_URL for the local backend so the checkbox is visible during local and e2e runs; the value need not resolve, since no e2e submission reaches the sidecar without a configured drafting step.

Adds two e2e cases to intake-submit.spec.ts: the checkbox appears ticked by default, and a sender can uncheck it and still submit successfully. Both are written per the task-7 brief but not run in this session, since running Playwright requires the full local stack (database, backend, frontend dev server) which was not started.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/tests/e2e/intake-submit.spec.ts

@@ -73,4 +73,32 @@ test.describe('Sending in an item through a link', () => {
     // is not on the site.
     await expect(page.getByText(/Nothing is listed for sale until/)).toBeVisible();
   });
+
+  // Ticked by default, because that is the decision: most items look better
+  // cut out, and the reverse default would mean almost nobody got it.
+  test('offers to remove the background, already ticked', async ({ page }) => {
+    await page.goto(`/submit/${token}`);
+
+    const control = page.getByRole('checkbox', { name: /remove the background/i });
+    await expect(control).toBeVisible();
+    await expect(control).toBeChecked();
+  });
+
+  test('lets a sender turn it off and still send', async ({ page }) => {
+    await page.goto(`/submit/${token}`);
+
+    const png = Buffer.from(
+      'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8z8BQDwAEhQGAhKmMIQAAAABJRU5ErkJggg==',
+      'base64'
+    );
+    await page.setInputFiles('input[type="file"]', {
+      name: `${RUN}-nobg.png`,
+      mimeType: 'image/png',
+      buffer: png
+    });
+    await page.getByRole('checkbox', { name: /remove the background/i }).uncheck();
+    await page.getByRole('button', { name: 'Send' }).click();
+
+    await expect(page.getByRole('heading', { name: 'Thank you — it arrived' })).toBeVisible();
+  });
 });