style: unify token progress bars with subtle cyan opacity ramp

Replace the disparate gradient/orange/red color scheme with a single
soft sky-blue (rgb 56,189,248) that subtly intensifies as usage grows.
Opacity ramps from 0.35 at low usage to 1.0 at full context.
Applies to both sidebar session bars and header token bar.

src/components/Header.tsx

@@ -66,11 +66,12 @@ export function Header({ status, sessionKey, onToggleSidebar, activeSessionData,
         const total = activeSessionData?.totalTokens || 0;
         if (!ctx) return null;
         const pct = Math.min(100, (total / ctx) * 100);
-        const barColor = pct > 95 ? 'bg-red-500' : pct > 80 ? 'bg-amber-500' : 'bg-gradient-to-r from-cyan-400 to-violet-500';
+        const opacity = Math.max(0.35, Math.min(1, pct / 100));
+        const barStyle = { width: `${pct}%`, backgroundColor: `rgba(56, 189, 248, ${opacity})` };
         return (
           <div className="px-4 py-1.5 bg-[#232329]/60 border-b border-white/8 flex items-center gap-3">
             <div className="flex-1 h-[5px] rounded-full bg-white/5 overflow-hidden">
-              <div className={`h-full rounded-full transition-all duration-500 ${barColor}`} style={{ width: `${pct}%` }} />
+              <div className="h-full rounded-full transition-all duration-500" style={barStyle} />
             </div>
             <span className="text-[11px] text-zinc-400 tabular-nums shrink-0 whitespace-nowrap">
               {(total / 1000).toFixed(1)}k / {(ctx / 1000).toFixed(0)}k tokens