feat(theme): define light-theme buttons (TactileButton glass)

The settings-page buttons (TactileButton = ImGui::Button + a glass overlay) looked
faint in light themes: --glass-fill was a 55-62% WHITE wash that paled the button
body, and --rim-light was a 6-10% (sometimes light-colored) edge that was invisible
on light surfaces, so buttons had no defined border. Across all light skins:
- --glass-fill white wash 0.55-0.62 -> 0.20 (button body no longer washed out;
  panels use the acrylic blur on capable systems, so only low-spec panel fills are
  affected).
- --rim-light -> each skin's on-surface tint @ 0.22 (a defined dark button edge
  instead of an invisible/light rim).
- --hover-overlay alpha -> 0.10 (visible hover).

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

res/themes/dune.toml

@@ -48,14 +48,14 @@ elevation = { --elevation-0 = "#FDF8F0", --elevation-1 = "#F5EDE0", --elevation-
 --chart-hover-ring = "rgba(176,120,64,0.28)"
 --tooltip-bg = "rgba(50,38,24,0.94)"
 --tooltip-border = "rgba(176,120,64,0.12)"
---glass-fill = "rgba(255,252,245,0.58)"
+--glass-fill = "rgba(255,252,245,0.20)"
 --glass-border = "rgba(176,120,64,0.14)"
 --glass-noise-tint = "rgba(180,140,80,0.03)"
 --tactile-top = "rgba(255,255,248,0.50)"
 --tactile-bottom = "rgba(255,255,248,0.08)"
---hover-overlay = "rgba(176,120,64,0.05)"
+--hover-overlay = "rgba(176,120,64,0.10)"
 --active-overlay = "rgba(176,120,64,0.10)"
---rim-light = "rgba(212,160,108,0.10)"
+--rim-light = "rgba(58,46,34,0.22)"
 --status-divider = "rgba(176,120,64,0.10)"
 --sidebar-hover = "rgba(176,120,64,0.08)"
 --sidebar-icon = "rgba(58,46,34,0.50)"