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/hooks/useEncounters.ts

@@ -4,6 +4,7 @@ import {
   updateEncounter,
   deleteEncounter,
   fetchEvolutions,
+  fetchForms,
 } from '../api/encounters'
 import type { CreateEncounterInput, UpdateEncounterInput } from '../types/game'
 
@@ -50,3 +51,11 @@ export function useEvolutions(pokemonId: number | null, region?: string) {
     enabled: pokemonId !== null,
   })
 }
+
+export function useForms(pokemonId: number | null) {
+  return useQuery({
+    queryKey: ['forms', pokemonId],
+    queryFn: () => fetchForms(pokemonId!),
+    enabled: pokemonId !== null,
+  })
+}