Add non-evolution form change support (Rotom, Oricorio, etc.)

Add a "Change Form" button in StatusChangeModal for Pokemon with
alternate forms sharing the same national_dex number. Mirrors the
existing evolution UI pattern, reusing currentPokemonId to track
the active form.

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

frontend/src/api/encounters.ts

@@ -4,6 +4,7 @@ import type {
   CreateEncounterInput,
   UpdateEncounterInput,
   Evolution,
+  Pokemon,
 } from '../types/game'
 
 export function createEncounter(
@@ -28,3 +29,7 @@ export function fetchEvolutions(pokemonId: number, region?: string): Promise<Evo
   const params = region ? `?region=${encodeURIComponent(region)}` : ''
   return api.get(`/pokemon/${pokemonId}/evolutions${params}`)
 }
+
+export function fetchForms(pokemonId: number): Promise<Pokemon[]> {
+  return api.get(`/pokemon/${pokemonId}/forms`)
+}