Add a backup media download progress bar

ts/components/ProgressBar.tsx

@@ -0,0 +1,21 @@
+// Copyright 2024 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import React from 'react';
+
+export function ProgressBar({
+  fractionComplete,
+}: {
+  fractionComplete: number;
+}): JSX.Element {
+  return (
+    <div className="ProgressBar">
+      <div
+        className="ProgressBar__fill"
+        style={{
+          marginInlineEnd: `${(1 - fractionComplete) * 100}%`,
+        }}
+      />
+    </div>
+  );
+}