add backup feature

src/routes/backup/+page.svelte

@@ -0,0 +1,23 @@
+<script>
+  import {getSharableUrl, stringifyCompressed} from "$lib/serial/serialization"
+  import {chords} from "$lib/serial/connection"
+
+  async function downloadBackup() {
+    const downloadUrl = URL.createObjectURL(await stringifyCompressed($chords))
+    const element = document.createElement("a")
+    element.setAttribute("download", "chords.chl")
+    element.href = downloadUrl
+    element.setAttribute("target", "_blank")
+    element.click()
+    URL.revokeObjectURL(downloadUrl)
+  }
+
+  async function createShareUrl() {
+    console.log(await getSharableUrl("chords", $chords))
+  }
+</script>
+
+<h1>Backup & Restore</h1>
+
+<button on:click={downloadBackup}><span class="icon">save</span> Backup</button>
+<button><span class="icon">settings_backup_restore</span> Restore</button>