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

backend/tests/test_ffmpeg_callbacks.py

@@ -26,8 +26,7 @@ class TestProgressWriter:
         writes = []
         clock = {"t": 1000.0}
         monkeypatch.setattr(worker, "_set_job", lambda job_id, **f: writes.append(f))
-        monkeypatch.setattr(worker.time, "monotonic", lambda: clock["t"])
-        write = worker._ffmpeg_progress_writer(7, "processing")
+        write = worker._ffmpeg_progress_writer(7, "processing", now=lambda: clock["t"])
         write(10)
         write(11)  # same second → dropped
         clock["t"] += 1.5
@@ -38,8 +37,7 @@ class TestProgressWriter:
         writes = []
         clock = {"t": 1000.0}
         monkeypatch.setattr(worker, "_set_job", lambda job_id, **f: writes.append(f))
-        monkeypatch.setattr(worker.time, "monotonic", lambda: clock["t"])
-        write = worker._ffmpeg_progress_writer(7, "processing")
+        write = worker._ffmpeg_progress_writer(7, "processing", now=lambda: clock["t"])
         write(10.1)
         clock["t"] += 5
         write(10.9)  # still 10% — the bar shows integers
@@ -50,7 +48,7 @@ class TestCancelPoll:
     def test_asks_once_then_reuses_the_answer(self, monkeypatch):
         calls = []
         monkeypatch.setattr(worker, "_job_status", lambda job_id: calls.append(job_id) or "running")
-        cancelled = worker._ffmpeg_cancel_poll(7)
+        cancelled = worker._cancel_poll(7)
         for _ in range(40):
             assert cancelled() is False
         assert len(calls) == 1
@@ -64,8 +62,7 @@ class TestCancelPoll:
             return "cancelled"
 
         monkeypatch.setattr(worker, "_job_status", status)
-        monkeypatch.setattr(worker.time, "monotonic", lambda: clock["t"])
-        cancelled = worker._ffmpeg_cancel_poll(7)
+        cancelled = worker._cancel_poll(7, now=lambda: clock["t"])
         assert cancelled() is True
         clock["t"] += 60  # cancellation is sticky: no re-query, and no un-cancelling
         assert cancelled() is True
@@ -75,8 +72,7 @@ class TestCancelPoll:
         clock = {"t": 1000.0}
         state = {"status": "running"}
         monkeypatch.setattr(worker, "_job_status", lambda job_id: state["status"])
-        monkeypatch.setattr(worker.time, "monotonic", lambda: clock["t"])
-        cancelled = worker._ffmpeg_cancel_poll(7)
+        cancelled = worker._cancel_poll(7, now=lambda: clock["t"])
         assert cancelled() is False
         state["status"] = "paused"
         clock["t"] += 1.0  # the throttle must not delay a cancel by more than its interval