Add optional pose/segmentation overlay copy to frame export

[gbeane]

Extends File → Export Frame with an option to also save an annotated copy of the frame.

Use case is to provide additional context when saving frames with pose or segmentation failures -- the raw frame can be used for labeling; the frame-with-overlay shows the failure.

What it does

• The export save dialog gains a checkbox: "Also save a copy with pose/segmentation overlay (…-overlay.png)". When checked, alongside <name>.png it writes <name>-overlay.png.
• The overlay copy is rendered at full native video resolution (matching the raw export) with:
  • Pose keypoints + skeleton for every identity.
  • Segmentation contours for every identity, when the pose file provides them (v6+); a no-op otherwise.
• Every identity is drawn the same — pose at full opacity, all segmentation contours in one color. Unlike the live view, the export has no "active identity" highlight.
• The checkbox state and the last-used directory are persisted in QSettings.

Implementation notes

• New pose_drawing.py factors the skeleton/keypoint drawing into a shared, coordinate-transform-parameterized helper. PoseOverlay (the live, display-resolution overlay) now uses it too, so the on-screen overlay and the export share one code path. Segmentation reuses the existing overlay_segmentation from jabs.video_reader.
• New PlayerWidget.get_overlay_frame() mirrors get_raw_frame() but bakes segmentation and paints the pose at native resolution.
• The save dialog now uses Qt's non-native dialog, since native OS dialogs can't host the checkbox.

Tests & docs

• New tests for the pose_drawing helper and the export handler's overlay path; the existing export-frame tests were updated for the new dialog.
• Full suite green (718 passed); lint and format clean.
• User guide updated in both copies (online docs/ and in-app help).

[Copilot]

Pull request overview

Extends the GUI’s File → Export Frame flow to optionally save a second “overlay” copy of the exported frame, rendered at native video resolution with pose skeleton/keypoints and (v6+) segmentation contours, while refactoring pose drawing into a shared helper used by both live overlay rendering and export.

Changes:

• Added shared pose_drawing.py helper and updated the live PoseOverlay to use it.
• Added PlayerWidget.get_overlay_frame() to render pose + segmentation overlays at native resolution for frame export.
• Updated the export-frame save dialog to a non-native QFileDialog with a persistent “also save overlay copy” checkbox; updated tests and user guide docs accordingly.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/ui/test_pose_drawing.py	Adds unit tests for the shared pose-drawing helper sizing and draw behavior.
tests/ui/test_menu_handlers.py	Updates export-frame tests for the new non-native dialog + checkbox and adds overlay-export test coverage.
src/jabs/ui/player_widget/pose_drawing.py	Introduces shared draw_identity_pose + native-resolution sizing helper.
src/jabs/ui/player_widget/player_widget.py	Adds get_overlay_frame() for native-res overlay rendering and exports.
src/jabs/ui/player_widget/overlays/pose_overlay.py	Refactors live overlay drawing to use the shared pose-drawing helper.
src/jabs/ui/main_window/menu_handlers.py	Adds checkbox-enabled export path for optional “-overlay.png” copy and persists checkbox state.
src/jabs/resources/docs/user_guide/gui.md	Updates in-app user guide text for the new export option.
docs/user-guide/gui.md	Updates online user guide text for the new export option.

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