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/i18n/locales/en/player.json

@@ -36,5 +36,7 @@
   "embedDisabledBody": "The owner disabled playback on other sites (common for auto-generated “Topic” music tracks). It still plays on YouTube.",
   "openOnYoutube": "Open on YouTube",
   "shortcutsHint": "Click: play/pause · Scroll or ↑ ↓: volume · F: fullscreen",
+  "nativeFullscreen": "YouTube fullscreen",
+  "nativeFullscreenHint": "YouTube fullscreen — the player's own controls (quality, subtitles, its fullscreen button). F fills the screen with our player instead.",
   "nativeControls": "YouTube controls active"
 }

frontend/src/i18n/locales/hu/player.json

@@ -36,5 +36,7 @@
   "embedDisabledBody": "A tulajdonos letiltotta a lejátszást más oldalakon (gyakori az auto-generált „Topic” zenei sávoknál). A YouTube-on viszont lejátszható.",
   "openOnYoutube": "Megnyitás YouTube-on",
   "shortcutsHint": "Kattintás: lejátszás/szünet · Görgő vagy ↑ ↓: hangerő · F: teljes képernyő",
+  "nativeFullscreen": "YouTube teljes képernyő",
+  "nativeFullscreenHint": "YouTube teljes képernyő — a lejátszó saját vezérlőivel (minőség, felirat, saját teljes képernyő gomb). Az F a mi lejátszónkkal tölti ki a képernyőt.",
   "nativeControls": "YouTube vezérlők aktívak"
 }