fix(player): the YouTube player's full window filled only its card — a blurred ancestor trapped the fixed stage (E28)

E28 S3d1 moved the modal's card from `.glass-card` (no blur) to the `.glass` panel role, whose
`backdrop-filter` makes the card the containing block of the fixed "full window" stage: F showed a
894×161 strip inside the card with a scrolling, wrapped control bar. While the stage is maximised the
card now drops its blur (`html .glass:has(> .player-stage--max)`, outranking the dark theme's rule);
measured live: the stage 1268×861 = the viewport, the blur back on exit.

frontend/src/index.css

@@ -1071,6 +1071,14 @@ html[data-theme="light"] {
      here escapes it — a higher level would claim an authority this layer neither has nor needs. */
   z-index: var(--z-index-rail);
 }
+/* …and it only fills the viewport while no ancestor has a filter: a `backdrop-filter` makes that
+   ancestor the fixed stage's containing block. The modal's card is a blurred `.glass` panel (E28 S3d1),
+   so while the stage is maximised the card drops its blur — the stage covers the card anyway. `html `
+   outranks `html[data-theme="dark"] .glass`, which sets the blur again (lib/playerStage.contract.test.ts). */
+html .glass:has(> .player-stage--max) {
+  backdrop-filter: none;
+  -webkit-backdrop-filter: none;
+}
 
 /* Thin, theme-aware scrollbars */
 * {

frontend/src/lib/playerStage.contract.test.ts

@@ -0,0 +1,19 @@
+import { readFileSync } from "node:fs";
+
+import { expect, it } from "vitest";
+
+// The YouTube player's "full window" (F) is a `position: fixed; inset: 0` stage INSIDE the modal's card.
+// A fixed element is sized by the viewport only while no ancestor has a filter: a `backdrop-filter`
+// makes that ancestor its containing block. E28 S3d1 moved the card from `.glass-card` (no blur) to
+// `.glass` (blurred), and the full window shrank to a 894×161 strip inside the card (E28 audit, seen
+// live). jsdom does no layout, so what is pinned is the escape rule: while the stage is maximised, the
+// card drops its blur, with a selector that outranks `html[data-theme="dark"] .glass` (0,2,1).
+const css = readFileSync(new URL("../index.css", import.meta.url), "utf8");
+
+it("a maximised player stage takes the blur off the glass card it sits in", () => {
+  const at = css.indexOf("html .glass:has(> .player-stage--max)");
+  expect(at, "the escape rule").toBeGreaterThan(css.indexOf('html[data-theme="dark"] .glass'));
+  const body = css.slice(at, css.indexOf("}", at));
+  expect(body).toMatch(/(^|[\s;{])backdrop-filter:\s*none/);
+  expect(body).toMatch(/-webkit-backdrop-filter:\s*none/);
+});