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/app/(mentor)/mentor/projects/[id]/page.tsx

@@ -596,7 +596,7 @@ function MilestonesSection({
               <div
                 key={milestone.id}
                 className={`flex items-start gap-3 p-3 rounded-lg border transition-all duration-200 hover:-translate-y-0.5 hover:shadow-sm ${
-                  isCompleted ? 'bg-green-50/50 border-green-200 dark:bg-green-950/20 dark:border-green-900' : ''
+                  isCompleted ? 'bg-green-50/50 border-green-200' : ''
                 }`}
               >
                 <Checkbox