Remove a background of one's own

- A cross on the tile's top-right corner removes an added background,
  critical-coloured at rest and named by its tooltip, as design draws it.
  The one in force offers none, since taking it away would leave nothing
  chosen; the view model refuses it too.
- The cross sits beside the item rather than in it, which would take the
  press first and choose the tile, and isn't a control of its own, so the
  menu keeps one keyboard model: Delete or Backspace on the tile does the
  same, and the tile says so with aria-keyshortcuts.
- Removed from IndexedDB and its picture released. End to end on
  Chromium and Firefox, a background removed by Delete is still gone
  after a reload.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

playwright/background-effects.spec.ts

@@ -326,6 +326,33 @@ test.describe("a background of one's own", () => {
     await addImage(page, red);
     await expect(mine).toBeVisible();
   });
+
+  test("a removed image stays removed", async ({ page }) => {
+    test.setTimeout(120_000);
+    await page.goto("/");
+    await SpaHelpers.createCall(
+      page,
+      "Removing user",
+      "Own background removed",
+    );
+    await addImage(page, await redImage(page));
+    const mine = page
+      .getByRole("group", { name: "Background effects" })
+      .getByRole("menuitemradio", { name: "Background 3" });
+    await expect(mine).toHaveAttribute("aria-keyshortcuts", "Delete");
+    await mine.focus();
+    await page.keyboard.press("Delete");
+    await expect(mine).toHaveCount(0);
+
+    await page.reload();
+    await expect(page.getByTestId("lobby_joinCall")).toBeVisible();
+    await page.getByRole("button", { name: "Camera", exact: true }).click();
+    await expect(
+      page.getByRole("group", { name: "Background effects" }),
+    ).toBeVisible();
+    await expect(mine).toHaveCount(0);
+    expect(await keptImages(page)).toEqual([]);
+  });
 });
 
 test.describe("what can be added", () => {