Fix: startActivity parameter type mismatch in useSessionHandlers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/app/vocab-worksheet/useSessionHandlers.ts

@@ -13,7 +13,7 @@ export async function startSessionFlow(params: {
   selectedMobileFile: { dataUrl: string; type: string; name: string } | null
   storedDocuments: StoredDocument[]
   ocrPrompts: OcrPrompts
-  startActivity: (type: string, meta: any) => void
+  startActivity: (type: string, meta?: any) => void
   setSession: (s: Session | null | ((prev: Session | null) => Session | null)) => void
   setWorksheetTitle: (t: string) => void
   setExtractionStatus: (s: string) => void