Keep background effects to desktops until the menu says what they cost

- The one support check asks for a desktop again, as #4059 had the
  pipeline do: menu and pipeline still agree, so a phone is neither
  offered blur nor refused it.
- Lifting it moves to the PR that tells the user where effects run
  slowly, so it is their choice to make.

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

src/components/CallFooterViewModel.test.ts

@@ -243,9 +243,9 @@ describe("createCallFooterViewModel", () => {
       );
     }
 
-    it("offers them on a phone whose browser can run them", () => {
+    it("offers nothing on a phone, where the pipeline would refuse them", () => {
       sdkSupportMock.mockReturnValue(true);
-      expect(lobbyOn("ios").toggleBlur$.value).toBeDefined();
+      expect(lobbyOn("ios").toggleBlur$.value).toBeUndefined();
     });
 
     it("offers them where the pipeline will honour them", () => {