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/Sidebar.tsx

@@ -120,11 +120,12 @@ export function Sidebar({ sessions, activeSession, onSwitch, open, onClose }: Pr
                   {(() => {
                     if (!s.contextTokens) return null;
                     const pct = Math.min(100, ((s.totalTokens || 0) / s.contextTokens) * 100);
-                    const barColor = pct > 95 ? 'bg-red-500' : pct > 80 ? 'bg-amber-500' : 'bg-gradient-to-r from-cyan-400 to-violet-500';
+                    const barOpacity = Math.max(0.35, Math.min(1, pct / 100));
+                    const barStyle = { width: `${pct}%`, backgroundColor: `rgba(56, 189, 248, ${barOpacity})` };
                     return (
                       <div className="flex items-center gap-1.5 mt-1">
                         <div className="flex-1 h-[3px] rounded-full bg-white/5 overflow-hidden">
-                          <div className={`h-full rounded-full ${barColor}`} style={{ width: `${pct}%` }} />
+                          <div className="h-full rounded-full" style={barStyle} />
                         </div>
                         <span className="text-[9px] text-zinc-500 tabular-nums shrink-0">{Math.round(pct)}%</span>
                       </div>