feat(mac): show context usage bars

apps/macos/Sources/Clawdis/ContextUsageBar.swift

@@ -0,0 +1,49 @@
+import SwiftUI
+
+struct ContextUsageBar: View {
+    let usedTokens: Int
+    let contextTokens: Int
+    var height: CGFloat = 6
+
+    private var clampedFractionUsed: Double {
+        guard self.contextTokens > 0 else { return 0 }
+        return min(1, max(0, Double(self.usedTokens) / Double(self.contextTokens)))
+    }
+
+    private var percentUsed: Int? {
+        guard self.contextTokens > 0, self.usedTokens > 0 else { return nil }
+        return min(100, Int(round(self.clampedFractionUsed * 100)))
+    }
+
+    private var tint: Color {
+        guard let pct = self.percentUsed else { return .secondary }
+        if pct >= 95 { return Color(nsColor: .systemRed) }
+        if pct >= 80 { return Color(nsColor: .systemOrange) }
+        if pct >= 60 { return Color(nsColor: .systemYellow) }
+        return Color(nsColor: .systemGreen)
+    }
+
+    var body: some View {
+        GeometryReader { proxy in
+            let fillWidth = proxy.size.width * self.clampedFractionUsed
+            ZStack(alignment: .leading) {
+                Capsule()
+                    .fill(Color.secondary.opacity(0.25))
+                Capsule()
+                    .fill(self.tint)
+                    .frame(width: fillWidth)
+            }
+        }
+        .frame(height: self.height)
+        .accessibilityLabel("Context usage")
+        .accessibilityValue(self.accessibilityValue)
+        .drawingGroup()
+    }
+
+    private var accessibilityValue: String {
+        if self.contextTokens <= 0 { return "Unknown context window" }
+        let pct = Int(round(self.clampedFractionUsed * 100))
+        return "\(pct) percent used"
+    }
+}
+