fix: replace the settings gear with a symmetric, concentric icon (#301)

The shipped gear was malformed, not merely off-centre. Measured from the path:
body spanned y 1.00-11.04 (centre 6.02) against a hub fixed at y=8, so the hub
sat ~2 units low in a 16-unit frame, the bottom third was empty, and the teeth
were uneven because the arcs were hand-tuned rather than derived.

Replaced with geometry generated on a polar grid about (8,8), making
concentricity structural. Six candidates were compared at true 16px, where the
differences that matter appear — at 120px all six looked fine.

Verified numerically as well as visually: 32 vertices, no duplicates, bbox
centre exactly (8.000, 8.000) matching the hub, max reach 7.66 including
half-stroke against an 8-unit half-frame.

v3.3.0