fix(player): the YouTube player keeps its size on a short window and when maximised

Two layout bugs since the player grew a side panel, both reported from prod.

Maximised (F) on a wide window, the video was a thin line across the screen: the side-panel layout
aligns the stage `self-start`, and Chrome applies a flex item's self-alignment to an absolutely
positioned box too, so the fixed stage took its content's height — 0 px. The maximised stage now
stretches between its insets whatever the layout asks of it.

On a short window under 1280 px (a laptop at 100%, a 1080p screen at 175%) only the side strips were
left: the card narrows so the chrome under the video fits, but that chrome grows as the card narrows
(its text wraps), and the loop ran the card down to 2 px. The video now never goes narrower than
36rem to make room; past that the card scrolls.

Measured before/after (localdev, real YouTube iframe): 1097×520 open 0×0 → 610×343; 1920×911 maximised
1920×0 → 1920×911; 1440×760 and 1366×650 maximised fill the window; leaving maximise restores the card.
Held by an e2e test at both sizes (red on the old code: 0 px wide, and 911 ≠ 0), green on Chrome,
WebKit and iPad.

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

@@ -465,7 +465,9 @@ describe("PlayerModal — fits the window's height", () => {
       <PlayerModal video={A} queue={[A, B]} startIndex={0} onClose={() => {}} onState={() => {}} />,
     );
     const row = document.querySelector<HTMLElement>(".player-stage")!.parentElement!.parentElement!;
-    expect(row.style.maxWidth).toBe("min(63rem, calc((100dvh - 3rem - 160px) * 16 / 9 + 7rem))");
+    expect(row.style.maxWidth).toBe(
+      "min(63rem, calc(max(36rem, (100dvh - 3rem - 160px) * 16 / 9) + 7rem))",
+    );
   });
 });
 
@@ -510,7 +512,7 @@ describe("PlayerModal — the side panel on a wide window", () => {
     expect(screen.queryByRole("button", { name: "player.description" })).toBeNull();
     // The row's width: the video at the window's height, plus the panel and the side strips.
     expect(card.parentElement!.style.maxWidth).toBe(
-      "calc((100dvh - 3rem - 0px) * 16 / 9 + 20rem + 7rem)",
+      "calc(max(36rem, (100dvh - 3rem - 0px) * 16 / 9) + 20rem + 7rem)",
     );
   });