Ads: pin courtside boards to the court's back edge, not a fixed baseline offset

The ad strip sat 12px above the far baseline, but the out-area lawn reaches z=-6, whose projected edge is ~15.5px above the baseline in portrait (camF is larger there) — so the lawn painted over the bottom of the boards. Pinning the strip's bottom to proj(0,0,-6).y clears them in both orientations.

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

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