Show video posters in the library by building a first-frame thumb when one is missing.

Co-authored-by: Cursor <cursoragent@cursor.com>

pages/index.vue

@@ -1717,8 +1717,7 @@
                     @click="openClip(entry.group.head)"
                   >
                     <span v-if="entry.group.head.hideThumbnail" class="sensitive-glow block h-full w-full" />
-                    <img v-else-if="entry.group.head.hasThumbnail" :src="`/api/library/clips/${entry.group.head.id}/thumb`" alt="" loading="lazy" decoding="async" class="h-full w-full object-cover">
-                    <span v-else class="block h-full w-full bg-zinc-800" />
+                    <img v-else :src="`/api/library/clips/${entry.group.head.id}/thumb`" alt="" loading="lazy" decoding="async" class="h-full w-full object-cover">
                   </button>
                   <div class="min-w-0 flex-1">
                     <div class="flex items-start gap-2">