fix: stat card left borders, tab shadow on toggle, larger rail icons

- Stat cards get 3px colored left border matching their icon color
- Guided/Chat toggle uses tab-active-shadow on selected state
- Rail icons increased to 24px with 1.6 stroke width
- Rail labels increased to 10px with font-sans (not mono)
- More vertical spacing between rail items

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/dashboard/PerformanceCards.tsx

@@ -71,7 +71,10 @@ export function PerformanceCards() {
           key={card.label}
           onClick={() => navigate(card.href)}
           className="card-interactive p-4 text-left fade-in"
-          style={{ animationDelay: `${400 + i * 60}ms` }}
+          style={{
+            animationDelay: `${400 + i * 60}ms`,
+            borderLeft: `3px solid ${card.iconColor}`,
+          }}
         >
           <div className="flex items-center justify-between mb-2">
             <p className="font-sans text-xs text-[0.5625rem] uppercase tracking-[0.1em] text-muted-foreground">