fix: mark-as-read for items from orphan channels

Items from channels with userId: null (created during earlier setup)
appeared in the unified timeline but had no _channelUid, causing the
mark-read JS handler to silently abort. Fall back to channelId (MongoDB
ObjectId) when channelUid is unavailable, and resolve it server-side
via getChannelById.

Confab-Link: http://localhost:8080/sessions/4d40ef89-a713-48c1-b4ed-0ffafca25677

views/partials/item-card.njk

@@ -203,6 +203,7 @@
             data-action="mark-read"
             data-item-id="{{ item._id }}"
             {% if item._channelUid %}data-channel-uid="{{ item._channelUid }}"{% endif %}
+            {% if item._channelId %}data-channel-id="{{ item._channelId }}"{% endif %}
             title="Mark as read">
       {{ icon("checkboxChecked") }}
       <span class="visually-hidden">Mark read</span>

views/timeline.njk

@@ -108,7 +108,8 @@
 
         const itemId = button.dataset.itemId;
         const channelUid = button.dataset.channelUid;
-        if (!itemId || !channelUid) return;
+        const channelId = button.dataset.channelId;
+        if (!itemId || (!channelUid && !channelId)) return;
 
         button.disabled = true;
 
@@ -116,7 +117,7 @@
           const formData = new URLSearchParams();
           formData.append('action', 'timeline');
           formData.append('method', 'mark_read');
-          formData.append('channel', channelUid);
+          formData.append('channel', channelUid || channelId);
           formData.append('entry', itemId);
 
           const response = await fetch(microsubApiUrl, {