perf(web): instant skips via gapless handoff to a pre-buffered element

Reintroduce the two-element handoff, this time as a skip-speed optimization
(not for the OS tile). A second <audio> element pre-buffers the next track;
on a forward skip playCurrent hands playback over to it instead of reloading
the source, so playback starts immediately instead of ~1s. Covers are
pre-warmed in parallel so the new artwork is ready too.

Listeners are bound to both elements once (handlers guard with el !== S.audio).
Safety: handoff only when the standby is buffered (readyState >= 2); otherwise
falls back to the unchanged source-swap path, and a failed handoff play()
recovers with a fresh source load. Web-only; the native bridge path is untouched.

Note: this does not change the OS media-tile flicker (a separate, MSE-only fix).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

index.html

@@ -742,6 +742,7 @@
 
 <div class="toast-container" id="toast-container"></div>
 <audio id="audio-element" preload="auto" playsinline crossorigin="anonymous"></audio>
+<audio id="audio-element-b" preload="auto" playsinline crossorigin="anonymous"></audio>
 <audio id="preview-audio"></audio>
 <audio id="abs-audio"></audio>