refactor: standardize error handling with toError() helper

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

server/src/engine/fight-loop.ts

@@ -1,7 +1,7 @@
 import { db, schema } from '../db/index.js'
 import { runMockFight } from './mock.js'
 import { eq } from 'drizzle-orm'
-import { pick } from '../lib/utils.js'
+import { pick, toError } from '../lib/utils.js'
 import { fightEvents } from './events.js'
 
 export interface FightResult {
@@ -137,7 +137,7 @@ export async function startFightLoop(options: FightLoopOptions = {}): Promise<vo
       }
     } catch (err) {
       if (onError) {
-        onError(err instanceof Error ? err : new Error(String(err)))
+        onError(toError(err))
       } else {
         console.error('[fight-loop] error:', err)
       }