refactor(ui): strip all dark: Tailwind classes (single-theme product)

Mechanical sweep of 41 files via `perl -i -pe 's{\s+dark:[\w:/\[\]\.\-]+}{}g'`.
All dark: variants were paired with light-mode counterparts already; no
elements relied on a dark:-only style.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/observer/dashboard/previous-round-section.tsx

@@ -52,7 +52,7 @@ export function PreviousRoundSection({ currentRoundId }: { currentRoundId: strin
         {!collapsed && (
           <CardContent className="space-y-4">
             {/* Headline Stat */}
-            <div className="flex items-center gap-3 rounded-lg bg-rose-50 dark:bg-rose-950/20 p-4">
+            <div className="flex items-center gap-3 rounded-lg bg-rose-50 p-4">
               <ArrowDown className="h-6 w-6 text-rose-500 shrink-0" />
               <div>
                 <p className="text-lg font-semibold">
@@ -85,7 +85,7 @@ export function PreviousRoundSection({ currentRoundId }: { currentRoundId: strin
                       </div>
                       <div className="relative h-2.5 rounded-full bg-muted overflow-hidden">
                         <div
-                          className="absolute inset-y-0 left-0 rounded-full bg-slate-300 dark:bg-slate-600 transition-all"
+                          className="absolute inset-y-0 left-0 rounded-full bg-slate-300 transition-all"
                           style={{ width: `${prevPct}%` }}
                         />
                         <div