feat: show rejoin link when bot is already in a fight

- Track active fight IDs per bot in orchestrator (Set → Map)
- Return fightId in "already in fight" error responses (409)
- Frontend shows "REJOIN FIGHT" link instead of generic error

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/queue.ts

@@ -1,7 +1,7 @@
 import { db, schema } from '../db/index.js'
 import { logger } from '../lib/logger.js'
 import { eq } from 'drizzle-orm'
-import { runFightAsync, isInFight } from './orchestrator.js'
+import { runFightAsync, isInFight, getActiveFightId } from './orchestrator.js'
 import { seedMockBots } from './mock.js'
 
 interface QueueEntry {
@@ -55,9 +55,12 @@ export async function joinQueue(botId: string): Promise<string> {
     throw new Error(`Cooldown active. Wait ${waitSec}s.`)
   }
 
-  // Check if already in a fight
+  // Check if already in a fight — return the fight ID so frontend can redirect
   if (isInFight(botId)) {
-    throw new Error('Bot is already in a fight.')
+    const activeFightId = getActiveFightId(botId)
+    const err = new Error('Bot is already in a fight.')
+    ;(err as any).fightId = activeFightId
+    throw err
   }
 
   // Load bot