feat(attachments): downloadable attachments in ProvenancePanel + PostInfoHeader

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/gallery/PostInfoHeader.vue

@@ -21,6 +21,11 @@
           v-if="info.data.post.url" :href="info.data.post.url"
           target="_blank" rel="noopener noreferrer"
         >↗ original post</a>
+        <a
+          v-for="at in (info.data.attachments || [])" :key="at.id"
+          class="fc-postinfo__attach"
+          :href="at.download_url" :download="at.original_filename"
+        >⬇ {{ at.original_filename }}</a>
       </div>
     </template>
     <span v-else-if="info && info.error" class="fc-postinfo__title">
@@ -90,4 +95,8 @@ function clear() {
   font-size: 13px; color: rgb(var(--v-theme-on-surface-variant));
 }
 .fc-postinfo__main a { color: rgb(var(--v-theme-accent)); text-decoration: none; }
+.fc-postinfo__attach {
+  font-size: 13px; color: rgb(var(--v-theme-accent));
+  text-decoration: none;
+}
 </style>