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.

server/api/admin/board-channels/[id].patch.js

@@ -14,6 +14,20 @@ export default defineEventHandler(async (event) => {
   if (body.slackChannelId !== undefined) updateData.slackChannelId = body.slackChannelId
   if (body.tagSlugs !== undefined) updateData.tagSlugs = body.tagSlugs
 
+  if (body.tagSlugs && body.tagSlugs.length) {
+    const conflict = await BoardChannel.findOne({
+      _id: { $ne: id },
+      tagSlugs: { $in: body.tagSlugs },
+    }).lean()
+    if (conflict) {
+      const taken = (conflict.tagSlugs || []).filter((s) => body.tagSlugs.includes(s))
+      throw createError({
+        statusCode: 409,
+        statusMessage: `Tag${taken.length > 1 ? 's' : ''} already mapped to "${conflict.name}": ${taken.join(', ')}`,
+      })
+    }
+  }
+
   try {
     const channel = await BoardChannel.findByIdAndUpdate(
       id,