Offer no effects for the session where the pipeline cannot be built

- A segmenter that can't be built, with the model unreachable or a GPU
  its delegate refuses, took the camera down with the pipeline: the SDK
  fails the track it was attaching to, and the lobby muted camera and
  microphone, on every load while the choice was kept.
- One is now built on trial when an effect is first wanted, before the
  pipeline is attached to anything, on the CPU where the GPU refuses.
  Where it can't be built, the options are disabled with the existing
  notice for the rest of the session. The choice is kept: a browser
  update may fix what failed.
- The view model takes that verdict from the pipeline's state rather than
  asking the SDK, so both say the same, and it no longer builds a WebGL
  context of its own to ask.
- Until the trial is over, the lobby opens no camera and the call makes no
  camera track, so neither starts without the effect.

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

playwright/background-effects.spec.ts

@@ -178,6 +178,46 @@ test.describe("background effects section", () => {
   });
 });
 
+test.describe("a pipeline that fails to build", () => {
+  test("offers no effects for the session, and keeps the choice", async ({
+    page,
+  }) => {
+    test.setTimeout(120_000);
+    await page.goto("/");
+    await SpaHelpers.createCall(page, "Failing user", "Pipeline fails");
+    const preview = page.locator("video").first();
+    const camera = await cameraColour(preview);
+    await page.route("**/*.tflite*", async (route) => route.abort());
+
+    await page.getByRole("button", { name: "Camera", exact: true }).click();
+    const tile = page.getByRole("menuitemradio", { name: "Background 1" });
+    await tile.click();
+    await expect(
+      page.getByText("Background effects are not supported on this platform."),
+    ).toBeVisible({ timeout: 30_000 });
+    await expect(tile).toBeDisabled();
+    await page.keyboard.press("Escape");
+    // The camera and microphone carry on without it.
+    expect(distance(await cameraColour(preview), camera)).toBeLessThan(30);
+    await expect(page.getByTestId("incall_mute")).toHaveAttribute(
+      "aria-checked",
+      "true",
+    );
+
+    await page.unroute("**/*.tflite*");
+    await page.reload();
+    await expect(page.getByTestId("lobby_joinCall")).toBeVisible();
+    await page.getByRole("button", { name: "Camera", exact: true }).click();
+    await expect(tile).toHaveAttribute("aria-checked", "true");
+    const picture = await averageColour(tile.locator("img"));
+    await expect
+      .poll(async () => distance(await averageColour(preview), picture), {
+        timeout: 60_000,
+      })
+      .toBeLessThan(distance(camera, picture) / 2);
+  });
+});
+
 test.describe("the chosen effect", () => {
   test("chosen effect survives leaving and rejoining", async ({ page }) => {
     test.setTimeout(120_000);