fix(player): give YouTube's own fullscreen a working path + round-6 findings

The reported bug: after F, the embed's own fullscreen button is dead. Root cause
measured in Chrome — the browser's fullscreen lock sits on OUR wrapper, so
YouTube's button would need a nested request from a cross-origin frame. Handing
the lock to the iframe fixes it, but `iframe.requestFullscreen()` only resolves
from a CLICK (from a key press the promise stays pending forever, and awaiting
it spends the activation so a fallback is denied too).

So: F keeps fullscreening our wrapper (unchanged, our overlays stay visible) and
a new toolbar button hands the lock to the iframe — verified in real Chrome:
document.fullscreenElement is the IFRAME at 1920x1024, i.e. a plain YouTube
fullscreen where its own controls work.

Round-6 review findings, all fixed:
- one _rate_limiter + one _cancel_poll shared by the yt-dlp hook and run_ffmpeg
  (was three hand-rolled throttles; cancel latency differed 2s vs 1s)
- stepVolume takes {delta, fallback} — two adjacent number args were silently
  transposable
- volume gestures before the player is ready are honoured instead of dropped
- normalizeVolume's fallback is typed unknown (its guard was unreachable) and
  applyVolume takes a number again
- the throttle tests inject a clock instead of patching global time.monotonic

frontend/src/lib/playerVolume.test.ts

@@ -34,24 +34,25 @@ describe("normalizeVolume", () => {
 
   it("uses the default when the fallback is unusable too", () => {
     expect(normalizeVolume(NaN, NaN)).toBe(DEFAULT_VOLUME);
-    expect(normalizeVolume(null, undefined as unknown as number)).toBe(DEFAULT_VOLUME);
+    expect(normalizeVolume(null, undefined)).toBe(DEFAULT_VOLUME);
+    expect(normalizeVolume(null, "80")).toBe(DEFAULT_VOLUME);
   });
 });
 
 describe("stepVolume", () => {
   it("steps up and down from the current level", () => {
-    expect(stepVolume(60, 5, 100)).toBe(65);
-    expect(stepVolume(60, -5, 100)).toBe(55);
+    expect(stepVolume(60, { delta: 5, fallback: 100 })).toBe(65);
+    expect(stepVolume(60, { delta: -5, fallback: 100 })).toBe(55);
   });
 
   it("clamps at both ends instead of wrapping", () => {
-    expect(stepVolume(98, 5, 100)).toBe(100);
-    expect(stepVolume(3, -5, 100)).toBe(0);
+    expect(stepVolume(98, { delta: 5, fallback: 100 })).toBe(100);
+    expect(stepVolume(3, { delta: -5, fallback: 100 })).toBe(0);
   });
 
   it("steps from the fallback when the player answers garbage", () => {
     // A spin must not lose notches to a stale/unusable reading — it continues from what we last set.
-    expect(stepVolume(undefined, -5, 90)).toBe(85);
-    expect(stepVolume(NaN, -5, 90)).toBe(85);
+    expect(stepVolume(undefined, { delta: -5, fallback: 90 })).toBe(85);
+    expect(stepVolume(NaN, { delta: -5, fallback: 90 })).toBe(85);
   });
 });