feat: add volume normalization using FFmpeg loudnorm filter

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

main.py

@@ -26,6 +26,18 @@ def playback_thread(file_path, state):
     stream = container.streams.audio[0]
     resampler = av.AudioResampler(format='s16', layout='stereo', rate=48000)
     
+    def build_graph():
+        graph = av.filter.Graph()
+        src = graph.add_abuffer(template=stream)
+        loudnorm = graph.add("loudnorm", "I=-16:TP=-1.5:LRA=11")
+        sink = graph.add("abuffersink")
+        src.link_to(loudnorm)
+        loudnorm.link_to(sink)
+        graph.configure()
+        return graph
+
+    graph = build_graph()
+    
     with open(fifo_path, 'wb') as fifo:
         iterator = container.decode(stream)
         while state.running:
@@ -41,6 +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
                 continue
                 
             if paused:
@@ -57,7 +70,14 @@ def playback_thread(file_path, state):
             with state.lock:
                 state.time_pos = float(frame.pts * stream.time_base)
                 
-            resampled_frames = resampler.resample(frame)
+            graph.push(frame)
+            while True:
+                try:
+                    filtered_frame = graph.pull()
+                except (av.AVError, BlockingIOError):
+                    break
+                    
+                resampled_frames = resampler.resample(filtered_frame)
                 for r_frame in resampled_frames:
                     arr = r_frame.to_ndarray()