[ui] spinner for video while waiting for poster to load

webui/src/components/display/BlobPreview.svelte

@@ -34,13 +34,24 @@
       <audio controls preload="auto" src="/api/raw/{address}" />
     {/if}
     {#if video}
-      <!-- svelte-ignore a11y-media-has-caption -->
-      <video
-        controls
-        preload="auto"
-        src="/api/raw/{address}"
-        poster="/api/thumb/{address}"
-      />
+      {#if imageLoaded != address }
+        <Spinner />
+
+        <img
+          src="/api/thumb/{address}"
+          alt={address}
+          on:load={() => (imageLoaded = address)}
+          on:error={() => (imageLoaded = address)}
+        />
+      {:else}
+        <!-- svelte-ignore a11y-media-has-caption -->
+        <video
+          controls
+          preload="auto"
+          src="/api/raw/{address}"
+          poster="/api/thumb/{address}"
+        />
+      {/if}
     {/if}
     {#if image}
       <a target="_blank" href="/api/raw/{address}?inline=1">