feat(board): redesign classifieds + Slack channel creation

Adds AdminGhost bot token for admin-only Slack channel creation, refreshes
BoardPostCard/Form layouts, and expands admin board-channels management.

app/pages/board.vue

@@ -1,5 +1,5 @@
 <template>
-  <PageShell title="Board" :subtitle="pageSubtitle">
+  <PageShell title="Bulletin Board" :subtitle="pageSubtitle">
     <!-- Action bar -->
     <div class="action-bar">
       <button type="button" class="new-post-btn" @click="openNewForm">
@@ -71,6 +71,7 @@
             :key="post._id"
             :post="post"
             :channels="channels"
+            :tags="cooperativeTags"
             :editable="isAuthor(post)"
             @edit="handleEdit"
             @delete="handleDelete"
@@ -302,17 +303,27 @@ onMounted(async () => {
 
 /* ---- FORM WRAPPER ---- */
 .form-wrapper {
-  padding: 20px 24px;
+  padding: 16px 24px;
   border-bottom: 1px dashed var(--border);
+  max-width: 640px;
 }
 
-/* ---- POST GRID ---- */
+/* ---- POST GRID (masonry via CSS columns) ---- */
 .post-grid {
-  display: grid;
-  grid-template-columns: repeat(2, 1fr);
-  gap: 16px;
+  column-count: 2;
+  column-gap: 16px;
   padding: 20px 24px;
 }
+.post-grid > * {
+  display: block;
+  width: 100%;
+  margin: 0 0 16px;
+}
+@media (min-width: 1400px) {
+  .post-grid {
+    column-count: 3;
+  }
+}
 
 /* ---- LOADING / EMPTY ---- */
 .loading-state {
@@ -340,7 +351,7 @@ onMounted(async () => {
 /* ---- RESPONSIVE ---- */
 @media (max-width: 1024px) {
   .post-grid {
-    grid-template-columns: 1fr;
+    column-count: 1;
   }
 }
 @media (max-width: 768px) {