Fix Opus duration reporting so seekbar and visualizer work

Surface the sidecar duration on the first Opus chunk instead of gating it on the first decoded buffers; C# locks UI Duration on chunk 1, and async WebCodecs decode left it at 0 — killing seek and the duration-gated visualizer.

DeepDrftPublic/Interop/audio/AudioPlayer.ts

@@ -229,14 +229,21 @@ export class AudioPlayer {
             const decoder = this.opusDecoder!;
             const buffers = await decoder.push(chunk);
 
+            // Duration is known up front from the sidecar — surface it as soon as the decoder reports it,
+            // NOT gated on the first decoded buffers. The C# layer locks Duration on the first chunk whose
+            // result carries a value (the `Duration == null` guard), and WebCodecs decode is async, so the
+            // earliest chunks can return zero buffers; gating duration on buffers means C# captures the
+            // initial 0 and never overwrites it — the WAV header path sets duration on chunk 1 because its
+            // header parses synchronously, which is the asymmetry this closes. Set once so a seek (which
+            // reinitialises the decoder) cannot overwrite it.
+            if (this.duration === 0 && decoder.totalDuration) {
+                this.duration = decoder.totalDuration;
+            }
+
             if (buffers.length > 0) {
                 for (const buffer of buffers) {
                     this.scheduler.addBuffer(buffer);
                 }
-                // Duration is known up front from the sidecar; set once (a seek must not overwrite it).
-                if (this.duration === 0 && decoder.totalDuration) {
-                    this.duration = decoder.totalDuration;
-                }
                 if (this.streamingStarted && this.isPlaying) {
                     this.scheduler.scheduleNewBuffers();
                 }