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.post.js

@@ -2,16 +2,51 @@ import BoardChannel from '../../models/boardChannel.js'
 import { requireAdmin } from '../../utils/auth.js'
 import { validateBody } from '../../utils/validateBody.js'
 import { boardChannelCreateSchema } from '../../utils/schemas.js'
+import { getSlackAdminService } from '../../utils/slack.ts'
 
 export default defineEventHandler(async (event) => {
   await requireAdmin(event)
 
   const body = await validateBody(event, boardChannelCreateSchema)
 
+  if (body.tagSlugs && body.tagSlugs.length) {
+    const conflict = await BoardChannel.findOne({ 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(', ')}`,
+      })
+    }
+  }
+
+  let slackChannelId = body.slackChannelId
+  let channelName = body.name
+
+  if (!slackChannelId) {
+    const slack = getSlackAdminService()
+    if (!slack) {
+      throw createError({
+        statusCode: 500,
+        statusMessage: 'Slack integration not configured',
+      })
+    }
+    try {
+      const created = await slack.createChannel(body.name)
+      slackChannelId = created.id
+      channelName = created.name
+    } catch (err) {
+      throw createError({
+        statusCode: 502,
+        statusMessage: `Failed to create Slack channel: ${err.data?.error || err.message}`,
+      })
+    }
+  }
+
   try {
     const channel = await BoardChannel.create({
-      name: body.name,
-      slackChannelId: body.slackChannelId,
+      name: channelName,
+      slackChannelId,
       tagSlugs: body.tagSlugs || []
     })