Handle Nincada split evolution (Ninjask + Shedinja)

When evolving Nincada, a confirmation prompt now offers to also add
Shedinja as a new encounter on the same route. The Shedinja encounter
uses a "shed_evolution" origin to bypass route-locking.

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

frontend/src/types/game.ts

@@ -119,6 +119,7 @@ export interface CreateEncounterInput {
   status: EncounterStatus
   catchLevel?: number
   isShiny?: boolean
+  origin?: string
 }
 
 export interface UpdateEncounterInput {