fix(watch): a link whose video is being made again says so, in the page's language

The public watch route answers video_preparing for an asset that is pending or
downloading, and the page shows "being prepared — try again" with a Retry
instead of "this link is gone". watch.html ships one lang; the page now sets
documentElement.lang to the language its strings were picked in.

frontend/src/components/WatchPage.dom.test.tsx

@@ -211,6 +211,26 @@ describe("WatchPage honest failures", () => {
     expect(page.queryByRole("button", { name: "Try again" })).toBeNull();
   });
 
+  it("a 404 whose video is being made again says to come back, with a retry", async () => {
+    window.history.replaceState({}, "", "/watch/tok-preparing");
+    vi.stubGlobal(
+      "fetch",
+      vi
+        .fn()
+        .mockResolvedValueOnce(res(404, { detail: { code: 404, reason: "video_preparing" } }))
+        .mockResolvedValue(
+          res(200, { needs_password: false, file_url: "/f.mp4", title: "A clip" }),
+        ),
+    );
+    const page = render(<WatchPage />);
+    const retry = await page.findByRole("button", { name: "Try again" });
+    expect(page.getByRole("status").textContent).toContain("This video is being prepared.");
+    expect(page.queryByText(GONE)).toBeNull();
+
+    fireEvent.click(retry);
+    await waitFor(() => expect(page.container.querySelector("video")).not.toBeNull());
+  });
+
   async function tryPassword(second: () => Promise<unknown>) {
     vi.stubGlobal(
       "fetch",