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.ts

@@ -16,15 +16,18 @@ const clamp = (n: number) => Math.max(0, Math.min(100, Math.round(n)));
  * being `0` (silently muting the player) is exactly the coercion to avoid. `fallback` is the last
  * level known to work; it is itself validated, so a corrupt value can never travel through it.
  */
-export function normalizeVolume(raw: unknown, fallback: number = DEFAULT_VOLUME): number {
+export function normalizeVolume(raw: unknown, fallback: unknown = DEFAULT_VOLUME): number {
   if (typeof raw === "number" && Number.isFinite(raw)) return clamp(raw);
   if (typeof fallback === "number" && Number.isFinite(fallback)) return clamp(fallback);
   return DEFAULT_VOLUME;
 }
 
-/** One volume step (wheel notch / arrow key) from `current`, falling back to `fallback` when the
- * player answered something unusable. */
-export function stepVolume(current: unknown, delta: number, fallback: number): number {
-  const base = normalizeVolume(current, fallback);
-  return clamp(base + delta);
+/** One volume step (wheel notch / arrow key) from `current`.
+ *
+ * `delta` and `fallback` ride in an options object on purpose: as two adjacent `number`
+ * parameters they were transposable without a type error, and swapping them silently steps the
+ * volume BY the current level. */
+export function stepVolume(current: unknown, opts: { delta: number; fallback: unknown }): number {
+  const base = normalizeVolume(current, opts.fallback);
+  return clamp(base + opts.delta);
 }