The thinking toggle (and other icon buttons) in the dashboard chat view
are visually identical in on/off states when using light mode.
Root cause: the light-mode .btn--icon rule sets background to near-white
(rgba 255,255,255,0.9) which has higher specificity than the generic
.btn.active orange tint (rgba 245,159,74,0.12), making the active state
invisible.
Fix:
1. Add :root[data-theme='light'] .btn--icon.active with explicit orange
background + border to override the white .btn--icon base.
2. Strengthen :root[data-theme='light'] .btn.active with border-color
so all button active states (not only icon buttons) are distinguishable
in light mode.
Closes#2832
448e87a84d