Replace fragile Dummy()-based cursor flow with a deterministic two-pass
layout system:
- Pass 1: compute exact Y positions for all elements (pure math)
- Pass 2: render at computed positions using SetCursorScreenPos + draw list
Eliminates the dual-coordinate mismatch that caused persistent centering
and overflow bugs. Height is computed once, not estimated then measured.
Also tune sidebar spacing via ui.toml:
- button-spacing: 4 → 6
- section-gap: 4 → 8
- Add section-label-pad-bottom (4px) below category labels
- bottom-padding: 0 → 4
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