add pwa functionality

src/lib/components/PwaStatus.svelte

@@ -0,0 +1,36 @@
+<script>
+  import {useRegisterSW} from "virtual:pwa-register/svelte"
+
+  const {needRefresh, updateServiceWorker, offlineReady} = useRegisterSW()
+</script>
+
+{#if !$needRefresh}
+  <button title="Update ready" class="icon" on:click={() => updateServiceWorker(true)}>update</button>
+{:else if $offlineReady}
+  <div title="App can now be used offline" class="icon">offline_pin</div>
+{:else}
+  <div title="Waiting to be offline ready..." class="icon working">offline_bolt</div>
+{/if}
+
+<style lang="scss">
+  button {
+    cursor: pointer;
+    color: var(--md-sys-color-on-background);
+    background: transparent;
+    border: none;
+  }
+
+  @keyframes working {
+    from {
+      opacity: 0;
+    }
+
+    to {
+      opacity: 1;
+    }
+  }
+
+  .working {
+    animation: working 1s ease alternate-reverse infinite;
+  }
+</style>