feat(theme): overarching light-theme contrast (buttons, dividers, accent bars)

From the theme sweep review, three overarching light-theme fixes:
- Buttons: bump ImGuiCol_Button light overlays 13/20/28% -> 16/23/30% so the
  small/secondary buttons read as clearly as the main ones.
- Dividers + outlines: raise --divider (0.12-0.14 -> 0.20) and --outline
  (-> 0.24) across all light skins so section/list dividers are visible on white.
- Ratio/balance bars: deepen the muted --success/--warning greens/ambers in the
  light skins that were pale (light, marble, dune) so the shielded/transparent
  bars + success text are vivid (color-pop-light + iridescent were already vivid).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/themes/iridescent.toml

@@ -28,8 +28,8 @@ elevation = { --elevation-0 = "#FDFBFF", --elevation-1 = "#F5F0FA", --elevation-
 --on-success = "#FFFFFF"
 --warning = "#E65100"
 --on-warning = "#000000"
---divider = "rgba(120,80,160,0.12)"
---outline = "rgba(120,80,160,0.14)"
+--divider = "rgba(120,80,160,0.20)"
+--outline = "rgba(120,80,160,0.24)"
 --scrim = "rgba(20,10,30,0.45)"
 --surface-hover = "rgba(140,107,175,0.06)"
 --surface-alt = "rgba(140,107,175,0.03)"