Add web interface and tests

packages/web/src/components/VaultPicker.tsx

@@ -0,0 +1,19 @@
+import { useCallback } from "react"
+
+export const VaultPicker: React.FC<{
+  setHandle(handle: FileSystemDirectoryHandle): void
+}> = ({ setHandle }) => {
+  const onClick = useCallback(async () => {
+    try {
+      const handle = await showDirectoryPicker()
+      setHandle(handle)
+    } catch (e) {
+      if ((e as Error).name === "AbortError") {
+        return
+      }
+      alert(e)
+    }
+  }, [setHandle])
+
+  return <button onClick={onClick}>Pick a vault here.</button>
+}