feat(ui): light/dark theme toggle and light mode color support

Add ThemeContext with localStorage persistence and FOUC-preventing
inline script. Add theme toggle button in sidebar. Update status
badges, toast notifications, live indicators, and approval cards
with dark: prefixed classes for proper light mode rendering.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/SidebarAgents.tsx

@@ -118,7 +118,7 @@ export function SidebarAgents() {
                       <span className="animate-ping absolute inline-flex h-full w-full rounded-full bg-blue-400 opacity-75" />
                       <span className="relative inline-flex rounded-full h-2 w-2 bg-blue-500" />
                     </span>
-                    <span className="text-[11px] font-medium text-blue-400">
+                    <span className="text-[11px] font-medium text-blue-600 dark:text-blue-400">
                       {runCount} live
                     </span>
                   </span>