Comprehensive UX polish: fix 19 friction points across all pages

Addresses broken interactions (notification bell, browser dialogs),
missing feedback states (disabled buttons, pricing changes, paused
indicator), unclear affordances (research queue, model tuning, funding
requirements), and navigation gaps (hash routing, keyboard shortcuts,
clickable dashboard cards, sidebar grouping, tutorial hints).

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

apps/web/src/pages/LeaderboardPage.tsx

@@ -88,7 +88,16 @@ export function LeaderboardPage() {
 
       <div className="bg-surface-900 border border-surface-700 rounded-xl overflow-hidden">
         {loading ? (
-          <div className="p-8 text-center text-surface-500">Loading...</div>
+          <div className="p-4 space-y-3">
+            {[...Array(5)].map((_, i) => (
+              <div key={i} className="flex items-center gap-4 animate-pulse">
+                <div className="w-8 h-4 bg-surface-800 rounded" />
+                <div className="flex-1 h-4 bg-surface-800 rounded" />
+                <div className="w-16 h-4 bg-surface-800 rounded" />
+                <div className="w-12 h-4 bg-surface-800 rounded" />
+              </div>
+            ))}
+          </div>
         ) : entries.length === 0 ? (
           <div className="p-8 text-center text-surface-500">
             <Trophy size={48} className="mx-auto mb-4 opacity-50" />