feat: add utility functions with tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

main.py

@@ -1,7 +1,19 @@
+import os
 import sys
 from PyQt6.QtWidgets import QApplication, QMainWindow
 
 
+def build_export_path(folder: str, basename: str, counter: int) -> str:
+    filename = f"{basename}_{counter:03d}.mp4"
+    return os.path.join(folder, filename)
+
+
+def format_time(seconds: float) -> str:
+    m = int(seconds) // 60
+    s = seconds - m * 60
+    return f"{m}:{s:04.1f}"
+
+
 def main():
     app = QApplication(sys.argv)
     win = MainWindow()