feat: Extract audio area — exact-length audio slice from the playhead, save-as

A dedicated "♪ Extract audio" button on the transport row grabs an exact
length of audio (set via the adjacent length box, from the playhead) and opens
a Save As dialog. Output format follows the chosen extension — WAV (pcm_s16le),
MP3 (libmp3lame), FLAC, m4a/aac, ogg/opus — re-encoding as needed; unknown
extensions let ffmpeg pick from the container.

- core.ffmpeg.build_audio_clip_command(input, start, duration, out_path):
  fast-seek + exact -t duration + -vn, codec by extension. Verified end-to-end
  (wav/mp3/flac all land at exactly the requested duration).
- Timeline shows the audio area as a distinct teal dashed band spanning
  [cursor, cursor+length], updated live as the playhead or length changes, so
  you see exactly what will be extracted.
- Length + last save dir persist in QSettings; button enabled once a file loads.

Tests: 3 core (codec-by-extension, exact length, case-insensitive) + 2 GUI
(controls exist, band tracks cursor/length).

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

tests/test_ui_structure.py

@@ -243,3 +243,28 @@ def test_subprofile_button_visibility_exact_match(win):
     win._apply_subcat_visibility()
     assert btns["blowjob"].isHidden()
     assert not btns["clap"].isHidden()
+
+
+def test_extract_audio_controls_exist(win):
+    from PyQt6.QtWidgets import QPushButton, QDoubleSpinBox
+    assert isinstance(win._btn_extract_audio, QPushButton)
+    assert isinstance(win._spn_audio_len, QDoubleSpinBox)
+    # Disabled until a file is loaded.
+    assert not win._btn_extract_audio.isEnabled()
+
+
+def test_audio_region_tracks_cursor_and_length(win):
+    # The teal audio band spans [cursor, cursor + length]; changing the length
+    # or moving the cursor moves the band. Fake a loaded file so the guard in
+    # _update_audio_region passes.
+    win._file_path = "/x/video.mp4"
+    win._cursor = 10.0
+    win._spn_audio_len.setValue(4.0)     # fires _on_audio_len_changed
+    assert win._timeline._audio_region == (10.0, 14.0)
+    win._cursor = 20.0
+    win._update_audio_region()
+    assert win._timeline._audio_region == (20.0, 24.0)
+    # No file -> band cleared.
+    win._file_path = ""
+    win._update_audio_region()
+    assert win._timeline._audio_region is None