Draw mode: spline still looks like polygon — smooth handles not working

[zworkb]

Part of #1.

Problem

Despite #71 (auto-generate Bézier handles), the spline in Draw mode still looks like a polygon — straight line segments between points instead of smooth curves.

Possible Causes

1. compute_handles() may not be called or may fail silently
2. The handles may be computed but the overlay doesn't render them (toScreen projection issue)
3. The Az/Alt → RA/Dec conversion may distort the handle positions
4. Performance issue Draw mode: slow response time when adding points #79 may cause the handle computation to be skipped

Investigation

• Check if compute_handles() is called in _on_map_click_sync() in layout.py
• Log the handle values after computation
• Check if handles survive the RA/Dec conversion in overlay_sync.py
• Compare with Freehand mode which produces smooth curves

Expected

After clicking 3+ points in Draw mode, the path should be a smooth Bézier curve through all points, with automatically computed tangent handles — same quality as Freehand mode.