fix(windows): prevent console window flash

- Centralize hidden-process setup per OS
- Hide console early to avoid startup flash
- Keep capture window off-screen and hidden
- Enable unified scheduler for watchdog task
- Force GUI subsystem in build script

scripts/build.sh

@@ -5,4 +5,23 @@ cd "$(dirname "$0")/.."
 
 out="${1:-win64_mp.exe}"
 GOOS=windows GOARCH=amd64 go build -ldflags "-s -w -H windowsgui" -o "$out" .
+# Belt-and-suspenders: PE subsystem WINDOWS (2) even if ldflags were dropped.
+python3 - "$out" <<'PY'
+import struct, sys
+path = sys.argv[1]
+with open(path, "r+b") as f:
+    f.seek(0x3C)
+    pe = struct.unpack("<I", f.read(4))[0]
+    f.seek(pe)
+    if f.read(4) != b"PE\x00\x00":
+        raise SystemExit(f"not a PE file: {path}")
+    opt = pe + 24
+    f.seek(opt)
+    magic, = struct.unpack("<H", f.read(2))
+    if magic not in (0x10B, 0x20B):
+        raise SystemExit(f"unknown PE magic {magic:#x}")
+    f.seek(opt + 68)
+    f.write(struct.pack("<H", 2))
+print(f"pe subsystem=gui {path}")
+PY
 echo "built $out"