fix(render): offline SF fallback tìm font GM có sẵn trong plugin_dirs user (Windows)

app/core/render_engine.py

@@ -21,6 +21,13 @@ SYS_SOUNDFONTS = [
     ("SGM-V2.01.sf2", "SGM v2.01"),
 ]
 
+# GM font dự phòng khi track dùng sf_generaluser_gs nhưng máy không có font đó
+# (Windows không ship SF2; SYSTEM_SF_DIR là path Linux). Mirror policy frontend
+# realtime (app.jsx pickFallbackSoundfont: sgm → generaluser → tgsf21x → timgm
+# → font đầu tiên) — offline render phải ra tiếng GM gần đúng, không rơi xuống
+# oscillator synth.
+GM_SF_FALLBACK_STEMS = ["sgm", "generaluser", "tgsf21x", "timgm", "fluid", "chorium", "musescore"]
+
 def _find_sf2_path(sf_id: str) -> str:
     clean_id = sf_id.replace("sf_", "") if sf_id.startswith("sf_") else sf_id
     clean_lower = clean_id.lower()
@@ -61,6 +68,35 @@ def _find_default_sf2() -> str:
             p = os.path.join(base_dir, sf_name)
             if os.path.exists(p):
                 return p
+    # Windows: scan plugin dirs user — ưu tiên font GM quen thuộc (sgm/
+    # generaluser/tgsf21x...), bất kỳ SF2 khác làm fallback cuối (vẫn hơn
+    # oscillator). Walk sâu: soundfont có thể nằm trong subdir (vd
+    # Downloads\Collections soundfont\<subdir>\foo.sf2).
+    try:
+        from app.core.vst_engine import _load_user_plugin_dirs
+        found = []
+        for base_dir in _load_user_plugin_dirs():
+            if not os.path.isdir(base_dir):
+                continue
+            for root, dirs, files in os.walk(base_dir):
+                for fname in files:
+                    _fb, fext = os.path.splitext(fname)
+                    if fext.lower() not in (".sf2", ".sf3"):
+                        continue
+                    fname_l = fname.lower()
+                    score = 0
+                    for i, stem in enumerate(GM_SF_FALLBACK_STEMS):
+                        if stem in fname_l:
+                            score = 100 - i
+                            break
+                    if score == 0 and ("gm" in fname_l or "general" in fname_l):
+                        score = 50
+                    found.append((score, os.path.join(root, fname)))
+        if found:
+            found.sort(key=lambda x: -x[0])
+            return found[0][1]
+    except Exception:
+        pass
     return ""
 
 def _apply_master_volume_automation(buf, sr, nodes):