feat: show available margin in Add/Remove margin modal (Bounty #19)

src/components/AvailableMargin.tsx

@@ -0,0 +1,42 @@
+interface AvailableMarginProps {
+  availableMargin: number
+  usedMargin: number
+  totalBalance: number
+  onAddMargin?: () => void
+  onRemoveMargin?: () => void
+}
+
+export function AvailableMargin({ availableMargin, usedMargin, totalBalance, onAddMargin, onRemoveMargin }: AvailableMarginProps) {
+  const usedPercent = totalBalance > 0 ? (usedMargin / totalBalance) * 100 : 0
+
+  return (
+    <div className="space-y-3">
+      <div className="flex justify-between text-sm">
+        <span className="text-gray-500">Available Margin</span>
+        <span className="font-medium">{availableMargin.toFixed(2)} USDC</span>
+      </div>
+      <div className="w-full bg-gray-200 dark:bg-gray-700 rounded-full h-2">
+        <div
+          className="bg-blue-500 h-2 rounded-full transition-all"
+          style={{ width: `${Math.min(usedPercent, 100)}%` }}
+        />
+      </div>
+      <div className="flex justify-between text-xs text-gray-400">
+        <span>Used: {usedMargin.toFixed(2)}</span>
+        <span>Total: {totalBalance.toFixed(2)}</span>
+      </div>
+      <div className="flex gap-2">
+        {onAddMargin && (
+          <button onClick={onAddMargin} className="flex-1 py-2 text-sm bg-green-600 text-white rounded hover:bg-green-700">
+            + Add Margin
+          </button>
+        )}
+        {onRemoveMargin && (
+          <button onClick={onRemoveMargin} className="flex-1 py-2 text-sm bg-gray-200 dark:bg-gray-700 rounded hover:bg-gray-300 dark:hover:bg-gray-600">
+            − Remove Margin
+          </button>
+        )}
+      </div>
+    </div>
+  )
+}