feat: audio area length — remove the upper cap + step by 1s

The audio extract length is meant for visualizing/grabbing sequences that can
run minutes long, but the control capped it and stepped in fiddly 0.10s
increments. Raise the range to effectively unlimited (24h; ffmpeg stops cleanly
at end-of-file if the source is shorter) and make the arrows step 1s — typing
still allows sub-second precision. Widen the field for the larger values.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_ui_structure.py

@@ -251,6 +251,9 @@ def test_extract_audio_controls_exist(win):
     assert isinstance(win._spn_audio_len, QDoubleSpinBox)
     # Disabled until a file is loaded.
     assert not win._btn_extract_audio.isEnabled()
+    # Arrows step by 1s and there's no practical upper cap (long audio areas).
+    assert win._spn_audio_len.singleStep() == 1.0
+    assert win._spn_audio_len.maximum() >= 3600.0
 
 
 def test_audio_region_tracks_cursor_and_length(win):