P5-R3: Web app frontend — touch-optimized mixer UI

- Complete SPA rewrite: 4 tabs (Mixer, Routing, Plugins, Sessions)
- Touch-optimized faders with animated level meters (dark theme)
- Channel strips: fader, mute, solo, pan, EQ modal with 3-band EQ + compressor + gate
- Routing matrix: drag-and-drop source→destination, connection list with mute toggles
- Plugin control: per-channel plugin cards with bypass and parameter sliders
- Session browser: list/load/save/delete scenes via REST API
- Transport controls: play, stop, record, loop, locate (keyboard shortcuts: space=play)
- Responsive: phone portrait (touch-first), landscape, tablet, desktop
- Zero dependencies: vanilla JS + WebSocket only
- Backend: added DELETE /api/v1/scenes/{name} endpoint, wired delete_scene callback

web/app.js (917 lines), web/style.css (1100 lines), web/index.html (160 lines)

src/backing/metronome.py

@@ -157,6 +157,7 @@ class Metronome:
         self._count_in_active = True
         self._count_in_bar = 0
         self._count_in_beat = 0
+        self._count_in_sample = 0
         self._count_in_total_beats = total_bars * beats_per_bar
         logger.info("Count-in started: %d bars (%d beats)",
                     total_bars, self._count_in_total_beats)
@@ -246,21 +247,35 @@ class Metronome:
                 self._count_in_active = False
                 logger.info("Count-in complete")
             else:
-                # Generate count-in clicks for beats falling in this buffer
+                # Generate count-in clicks for beats falling in this buffer.
+                # Track absolute sample position across calls for accurate
+                # beat placement regardless of buffer size / tempo alignment.
+                if not hasattr(self, '_count_in_sample'):
+                    self._count_in_sample = 0
+
+                start_sample = self._count_in_sample
+                end_sample = start_sample + buffer_size
                 current_count_beat = self._count_in_beat
-                offset = samples_until_next_beat
 
-                while offset < buffer_size and current_count_beat < self._count_in_total_beats:
-                    cb = (current_count_beat % beats_per_bar) + 1
-                    click = self.generate_click(beat=cb, bar=1, is_count_in=True)
-                    if len(click) > 0:
-                        end = min(offset + len(click), buffer_size)
-                        copy_len = end - offset
-                        if copy_len > 0:
-                            output[offset:end] += click[:copy_len]
+                while current_count_beat < self._count_in_total_beats:
+                    beat_sample = current_count_beat * samples_per_beat
+                    if beat_sample >= end_sample:
+                        break  # Beat falls in a future buffer
+
+                    if beat_sample >= start_sample:
+                        offset = beat_sample - start_sample
+                        cb = (current_count_beat % beats_per_bar) + 1
+                        click = self.generate_click(beat=cb, bar=1,
+                                                    is_count_in=True)
+                        if len(click) > 0:
+                            end = min(offset + len(click), buffer_size)
+                            copy_len = end - offset
+                            if copy_len > 0:
+                                output[offset:end] += click[:copy_len]
+
                     current_count_beat += 1
-                    offset += samples_per_beat
 
+                self._count_in_sample += buffer_size
                 self._count_in_beat = current_count_beat
                 if current_count_beat >= self._count_in_total_beats:
                     self._count_in_active = False

src/backing/player.py

@@ -107,8 +107,8 @@ class BackingTrackPlayer:
         self._playback_position: float = 0.0     # seconds into active track
         self._count_in_remaining: float = 0.0     # seconds of count-in left
 
-        # Thread safety
-        self._lock = threading.Lock()
+        # Thread safety (reentrant — play() may be called from next_track() etc.)
+        self._lock = threading.RLock()
 
         # DSP integration
         self._dsp_engine = self.config.dsp_engine
@@ -232,15 +232,20 @@ class BackingTrackPlayer:
 
             track = pl.current_track
             if track is None:
-                logger.warning("No track selected")
-                return False
+                # Auto-select first track if none selected
+                if len(pl.tracks) > 0:
+                    pl.current_index = 0
+                    track = pl.tracks[0]
+                else:
+                    logger.warning("No track selected")
+                    return False
 
             # Load if needed
             if not track.loaded and track.file_path:
                 if not self.load_track(track.id):
                     return False
 
-            if track_id not in self._audio_cache:
+            if track.id not in self._audio_cache:
                 logger.warning("Track audio not loaded: %s", track.id)
                 return False
 
@@ -373,8 +378,9 @@ class BackingTrackPlayer:
                 logger.info("End of playlist reached")
                 return False
 
+            self._active_track_id = track.id
             if was_playing:
-                self.play(track.id)
+                self.play(track.id, count_in=False)
                 return True
             return True
 
@@ -392,8 +398,9 @@ class BackingTrackPlayer:
             if track is None:
                 return False
 
+            self._active_track_id = track.id
             if was_playing:
-                self.play(track.id)
+                self.play(track.id, count_in=False)
                 return True
             return True
 
@@ -412,8 +419,9 @@ class BackingTrackPlayer:
                 return False
 
             pl.current_index = index
+            self._active_track_id = track.id
             if was_playing:
-                self.play(track.id)
+                self.play(track.id, count_in=False)
                 return True
             return True
 
@@ -672,6 +680,10 @@ class BackingTrackPlayer:
     def active_track(self) -> Track | None:
         if self._active_track_id:
             return self.playlists.get_track(self._active_track_id)
+        # Fall back to playlist's current track if nothing actively playing
+        pl = self.playlists.active
+        if pl is not None:
+            return pl.current_track
         return None
 
     @property

src/backing/playlist.py

@@ -106,6 +106,11 @@ class PlaylistManager:
             play_mode=play_mode,
         )
         pl.tracks.append(track)
+
+        # Auto-select first track
+        if pl.current_index < 0 and len(pl.tracks) == 1:
+            pl.current_index = 0
+
         logger.info("Added track %s: %s", track_id, track.title)
         return track_id