FIX: 3 bugs - soundfont engine fallback, media explorer icons/drag, carla midi playback routing

- render_engine: soundfont track không có plugin_id giờ vẫn dùng instrument sf2 thay vì rơi về wave synth
- fontawesome: sửa đường dẫn webfonts (bỏ ../) hết 404 icon
- media explorer: drag MIDI library items vào timeline tạo được midi item (sinh 6 MIDI loop mẫu + resolve drop file)
- app.jsx: 3 path gate window.SonicSF cho phép route playback qua SonicCarlaMidi

app/core/render_engine.py

@@ -270,7 +270,7 @@ class PythonRenderEngine:
                                 # pedalboard >= 0.9: instrument KHÔNG vào Pedalboard container
                                 synth_buffer = vst(midi_messages, sample_rate=self.sample_rate,
                                                    duration=total_needed / float(self.sample_rate), num_channels=2)
-                            elif instrument_id and (instrument_id.startswith("sf_") or soundfont_id):
+                            elif soundfont_id or (instrument_id and instrument_id.startswith("sf_")):
                                 sf_path = _find_sf2_path(soundfont_id or instrument_id)
                                 # 3-level fallback: selected SF → default SF → oscillator synth
                                 if not sf_path or not os.path.exists(sf_path) or not HAS_PYFLUIDSYNTH: