feat(player): the OS media controls, the basic keys and resume for the library and shared-link players

Plex, library and /watch tell the operating system what plays (title, line, picture) and take its
play / pause / skip / seek through one hook (lib/mediaSession); the Plex player drives it with its own
seek and clock, so the lock screen shows the film's position, not the stream's. The YouTube player is
left alone: its iframe already publishes all of it, and the OS routes to the frame that plays.

The library and /watch players get the basic keys (lib/mediaKeys, the keymap's mediaPlayer rows) and
open where the viewer stopped (lib/resume): the library keeps the point on the server, per user and
library item (library_states, POST /api/downloads/{id}/progress, listed back on both listings), the
shared link in this browser. A started library row continues, its menu can start it over, and its
picture carries the cards' progress bar. The resume rule (too early / near the end = from the start)
is one function shared with the YouTube route.

The library dialog opens focused on its panel (Modal initialFocus "player"): on the ✕, Space closed
the film. While the media element itself has the focus, Space and the arrows stay its own controls'
— the browser acts on them whatever a listener does, and one Space played and paused.

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

@@ -286,3 +286,41 @@ describe("WatchPage honest failures", () => {
     );
   });
 });
+
+// A shared link opens where THIS browser stopped in it (the viewer has no account), and keeps the
+// point as it plays — per link, so another link starts at its own place.
+describe("WatchPage resume", () => {
+  function loaded(video: HTMLVideoElement, duration = 600) {
+    let t = 0;
+    Object.defineProperties(video, {
+      currentTime: { get: () => t, set: (v: number) => void (t = v), configurable: true },
+      duration: { get: () => duration, configurable: true },
+      readyState: { get: () => 1, configurable: true },
+    });
+    video.dispatchEvent(new Event("loadedmetadata"));
+    return video;
+  }
+
+  it("keeps where it stopped, and opens there on the next visit", async () => {
+    window.history.replaceState({}, "", "/watch/tok-resume");
+    mockMeta({ needs_password: false, audio: false, file_url: "/f.mp4", title: "A clip" });
+    const first = render(<WatchPage />);
+    await waitFor(() => expect(first.container.querySelector("video")).not.toBeNull());
+    const video = loaded(first.container.querySelector("video")!);
+    expect(video.currentTime).toBe(0);
+    video.dispatchEvent(new Event("playing"));
+    video.currentTime = 250;
+    video.dispatchEvent(new Event("pause"));
+    cleanup();
+
+    const second = render(<WatchPage />);
+    await waitFor(() => expect(second.container.querySelector("video")).not.toBeNull());
+    expect(loaded(second.container.querySelector("video")!).currentTime).toBe(250);
+    cleanup();
+
+    window.history.replaceState({}, "", "/watch/tok-other");
+    const other = render(<WatchPage />);
+    await waitFor(() => expect(other.container.querySelector("video")).not.toBeNull());
+    expect(loaded(other.container.querySelector("video")!).currentTime).toBe(0);
+  });
+});