feat(admin/members): mark-as-Slack-invited button + date display

Replaces the placeholder Slack-invite handler with a call to the new
PATCH /api/admin/members/:id/slack-status endpoint. Status labels are
reworded to match reality (no Slack API call from this app):

- Pending → Not yet invited
- Invited → Invited <slackInvitedAt>
- Action button copy → 'Mark as Slack invited'
- Removes slackInviteStatus reads from the member detail page (the
  remaining repo-wide sweep lands in the cleanup task).

app/pages/admin/members/index.vue

@@ -124,8 +124,11 @@
               </span>
             </td>
             <td>
-              <span :class="member.slackInvited ? 'status-ok' : 'status-dim'">
-                {{ member.slackInvited ? "Invited" : "Pending" }}
+              <span v-if="member.slackInvited" class="status-ok">
+                Invited {{ formatDate(member.slackInvitedAt) }}
+              </span>
+              <span v-else class="status-dim">
+                Not yet invited
               </span>
             </td>
             <td class="col-mono col-date">
@@ -135,8 +138,12 @@
               <NuxtLink :to="`/admin/members/${member._id}`" class="link-btn" @click.stop
                 >View</NuxtLink
               >
-              <button class="link-btn" @click.stop="sendSlackInvite(member)">
-                Slack
+              <button
+                v-if="!member.slackInvited"
+                class="link-btn"
+                @click.stop="markSlackInvited(member)"
+              >
+                Mark as Slack invited
               </button>
               <button class="link-btn" @click.stop="editMember(member)">Edit</button>
             </td>
@@ -829,8 +836,24 @@ const submitInvites = async () => {
 };
 
 // --- Existing actions ---
-const sendSlackInvite = (member) => {
-  console.log("Send Slack invite to:", member.email);
+const markSlackInvited = async (member) => {
+  try {
+    const res = await $fetch(
+      `/api/admin/members/${member._id}/slack-status`,
+      {
+        method: "PATCH",
+        body: { slackInvited: true },
+      },
+    );
+    Object.assign(member, res.member);
+    toast.add({ title: "Marked as Slack invited", color: "success" });
+  } catch (err) {
+    toast.add({
+      title: "Failed to mark Slack invited",
+      description: err.data?.statusMessage || err.message,
+      color: "error",
+    });
+  }
 };
 
 // --- Edit Member ---