Add endpoint mode options, release channel rollback handling, and runtime refresh improvements

- Implement `endpoint_mode` (manual/auto) and related configuration in i18n across multiple locales.
- Add new utility methods for channel rollback and runtime confirmation workflows.
- Update unit and E2E tests to cover runtime refresh behavior, channel switching mechanics, and release references.
- Enhance `SessionUser` and related services to improve error handling during release runtime refresh.
- Add data-test attributes for frontend components to support improved test coverage.

tests/e2e/release-channel-unavailable.spec.js

@@ -122,6 +122,61 @@ const runtimeWithSelectableReleaseDetails = () => ({
   ],
 });
 
+const runtimeWithFailingBetaSwitch = () => ({
+  generated_at: "2026-05-19T09:10:00.000Z",
+  trace_id: "trace-stable-channel",
+  channel: {
+    id: 1,
+    slug: "stable",
+    name: "Stable",
+    description: "Standard production channel.",
+    enabled: true,
+    default_channel: true,
+  },
+  versions: {
+    frontend: { version_label: "frontend-stable", commit_sha: "abc123" },
+    api: { version_label: "api-stable", commit_sha: "def456" },
+    bundle_id: 31,
+  },
+  availability: {
+    configured: true,
+    missing: [],
+    status: "ready",
+  },
+  available_channels: [
+    {
+      channel: {
+        id: 1,
+        slug: "stable",
+        name: "Stable",
+        description: "Standard production channel.",
+        enabled: true,
+        default_channel: true,
+      },
+      availability: {
+        configured: true,
+        missing: [],
+        status: "ready",
+      },
+    },
+    {
+      channel: {
+        id: 3,
+        slug: "beta",
+        name: "Beta",
+        description: "Beta validation channel.",
+        enabled: true,
+        default_channel: false,
+      },
+      availability: {
+        configured: true,
+        missing: [],
+        status: "ready",
+      },
+    },
+  ],
+});
+
 async function boot(page, runtime = unavailableRuntime, locale = "en") {
   await page.addInitScript((selectedLocale) => {
     window.localStorage.setItem("locale", selectedLocale);
@@ -283,6 +338,40 @@ test("release channel choices show git commit and release time when available",
   );
 });
 
+test("failed sidebar release channel switches keep the previous channel active", async ({ page }, testInfo) => {
+  test.skip(testInfo.project.name.includes("mobile"), "The sidebar release selector is hidden in the mobile layout.");
+
+  const runtime = runtimeWithFailingBetaSwitch();
+  await boot(page, runtime);
+  const runtimeRequests = [];
+
+  await page.route("**/release/runtime**", async (route) => {
+    const url = new URL(route.request().url());
+    const selectedChannel = url.searchParams.get("release_channel") || "";
+    runtimeRequests.push(selectedChannel || "default");
+    if (selectedChannel === "beta") {
+      await route.fulfill(json({ data: { message: "Beta runtime unavailable" } }, 502));
+      return;
+    }
+    await route.fulfill(json({ data: runtime }));
+  });
+
+  await page.goto("/user", { waitUntil: "domcontentloaded" });
+  const stable = page.getByTestId("release-channel-option-stable");
+  const beta = page.getByTestId("release-channel-option-beta");
+
+  await expect(stable).toBeVisible({ timeout: GUARD_TIMEOUT_MS });
+  await expect(stable).toHaveAttribute("aria-pressed", "true");
+  await expect(beta).toHaveAttribute("aria-pressed", "false");
+
+  await beta.click();
+  await expect(stable).toHaveAttribute("aria-pressed", "true");
+  await expect(beta).toHaveAttribute("aria-pressed", "false");
+  await expect(page.getByRole("alert")).toContainText("previous channel is still active");
+  await expect.poll(() => page.evaluate(() => window.localStorage.getItem("release_channel_selected_slug"))).toBeNull();
+  expect(runtimeRequests).toContain("beta");
+});
+
 test("predefined release channel text is localized on the guard page", async ({ page }) => {
   const internalRuntime = {
     ...unavailableRuntime,