fix(standalone): bundle pedalboard.midi_utils + pyfluidsynth dll path

- engine.spec hiddenimports: pedalboard.midi_utils (C++ ExternalPlugin.h
  import dong, PyInstaller khong phat hien duoc -> midi-render 500)
- engine.spec hiddenimports: fluidsynth (pyfluidsynth cffi) cho
  /soundfont-render native
- vst_engine.py: _ensure_fluidsynth_dll_on_path() - frozen app them
  exe_dir vao PATH de ctypes.util.find_library tim thay
  libfluidsynth-3.dll dat ke exe

app/core/vst_engine.py

@@ -5,6 +5,22 @@ import numpy as np
 import functools
 from ctypes import c_char_p
 
+
+def _ensure_fluidsynth_dll_on_path():
+    """Frozen (PyInstaller): pyfluidsynth tim dll qua ctypes.util.find_library
+    (chi quet PATH). Dat libfluidsynth-3.dll ke exe roi them exe_dir vao PATH
+    truoc khi import fluidsynth - nguoc lai: 'Couldn't find the FluidSynth
+    library' -> /soundfont-render tra 501."""
+    import sys
+    if getattr(sys, 'frozen', False):
+        exe_dir = os.path.dirname(os.path.abspath(sys.executable))
+        if exe_dir and exe_dir not in os.environ.get('PATH', '').split(os.pathsep):
+            os.environ['PATH'] = exe_dir + os.pathsep + os.environ.get('PATH', '')
+
+
+_ensure_fluidsynth_dll_on_path()
+
+
 def midi_note_to_freq(note_number: int) -> float:
     return 440.0 * (2.0 ** ((note_number - 69) / 12.0))