fix(ui): improve active button contrast in light mode (#2832)

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

ui/src/styles/chat/layout.css

@@ -344,6 +344,15 @@
   color: var(--text);
 }
 
+/* Active state for icon buttons in light mode — without this override the
+   white .btn--icon background (specificity) masks the generic .btn.active
+   orange tint, making on/off states visually identical.  See #2832. */
+:root[data-theme="light"] .btn--icon.active {
+  background: rgba(245, 159, 74, 0.18);
+  border-color: rgba(245, 159, 74, 0.5);
+  color: rgba(16, 24, 40, 0.9);
+}
+
 .btn--icon svg {
   display: block;
   width: 18px;

ui/src/styles/components.css

@@ -510,6 +510,11 @@
   border-color: var(--accent);
 }
 
+:root[data-theme="light"] .btn.active {
+  background: rgba(245, 159, 74, 0.14);
+  border-color: rgba(245, 159, 74, 0.45);
+}
+
 /* ===========================================
    Utilities
    =========================================== */