Mark the chosen point with a crosshair instead of a square

[enaboapps]

Closes #1017.

Summary

• After point scan chooses a point, the action menu and drag-destination scan now mark it with a crosshair instead of a 10 px outlined square. The crosshair has four 12 px arms in the scan colour and an 8 px clear centre, so the exact point stays visible.
• The crosshair uses the existing click-through, nonactivating strips, so Windows and macOS draw it the same way.
• At a display edge, the arms keep their full thickness on screen and are cut short at the edge. An arm too short to draw is left out.
• Removed the scan_menu::outline helper, which only drew the square.
• Updated docs/point-scan.md.

No change to settings, protocol, or saved schemas.

Validation

• npm run prediction-model, npm run lint, npm test (293 passed), npm run build
• cargo fmt --check, cargo clippy --all-targets -D warnings
• cargo test (737 passed), including the new a_chosen_point_is_marked_by_a_crosshair_that_stays_on_screen, which checks the marker mid-screen, at a corner, and that Home marks no point.
• Not yet checked by eye on Windows or macOS hardware.

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