refactor: rewrite sidebar layout with two-pass architecture

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

res/themes/ui.toml

@@ -1230,8 +1230,9 @@ width = { size = 140.0 }
 collapsed-width = { size = 64.0 }
 collapse-anim-speed = { size = 10.0 }
 auto-collapse-threshold = { size = 800.0 }
-section-gap = { size = 4.0 }
+section-gap = { size = 8.0 }
 section-label-pad-left = { size = 16.0 }
+section-label-pad-bottom = { size = 4.0 }
 item-height = { size = 36.0 }
 item-pad-x = { size = 8.0 }
 min-height = { size = 360.0 }
@@ -1248,8 +1249,8 @@ icon-half-size = { size = 7.0 }
 icon-label-gap = { size = 8.0 }
 badge-radius-dot = { size = 4.0 }
 badge-radius-number = { size = 8.0 }
-button-spacing = { size = 4.0 }
-bottom-padding = { size = 0.0 }
+button-spacing = { size = 6.0 }
+bottom-padding = { size = 4.0 }
 exit-icon-gap = { size = 4.0 }
 cutout-shadow-alpha = { size = 55 }
 cutout-highlight-alpha = { size = 8 }