fix: delay filter graph creation until first frame is decoded

Co-authored-by: aider (gemini/gemini-3.1-pro-preview) <aider@aider.chat>

main.py

@@ -26,9 +26,9 @@ def playback_thread(file_path, state):
     stream = container.streams.audio[0]
     resampler = av.AudioResampler(format='s16', layout='stereo', rate=48000)
     
-    def build_graph():
+    def build_graph(template_frame):
         graph = av.filter.Graph()
-        src = graph.add_abuffer(template=stream)
+        src = graph.add_abuffer(template=template_frame)
         loudnorm = graph.add("loudnorm", "I=-16:TP=-1.5:LRA=11")
         sink = graph.add("abuffersink")
         src.link_to(loudnorm)
@@ -36,7 +36,7 @@ def playback_thread(file_path, state):
         graph.configure()
         return graph
 
-    graph = build_graph()
+    graph = None
     
     with open(fifo_path, 'wb') as fifo:
         iterator = container.decode(stream)
@@ -53,7 +53,7 @@ def playback_thread(file_path, state):
                 with state.lock:
                     state.seek_request = None
                 iterator = container.decode(stream)
-                graph = build_graph()  # Reset filter graph on seek
+                graph = None  # Reset filter graph on seek
                 continue
                 
             if paused:
@@ -70,7 +70,18 @@ def playback_thread(file_path, state):
             with state.lock:
                 state.time_pos = float(frame.pts * stream.time_base)
                 
+            if graph is None:
+                try:
+                    graph = build_graph(frame)
+                except Exception as e:
+                    print(f"Filter graph error: {e}", file=sys.stderr)
+                    break
+                    
+            try:
                 graph.push(frame)
+            except av.AVError:
+                continue
+                
             while True:
                 try:
                     filtered_frame = graph.pull()