Add frontend API client and TanStack Query hooks

Install @tanstack/react-query, create a fetch-based API client with typed
functions for all endpoints, and add query/mutation hooks for games, pokemon,
runs, and encounters. Includes Vite dev proxy for /api and QueryClientProvider
setup.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/api/encounters.ts

@@ -0,0 +1,24 @@
+import { api } from './client'
+import type {
+  Encounter,
+  CreateEncounterInput,
+  UpdateEncounterInput,
+} from '../types/game'
+
+export function createEncounter(
+  runId: number,
+  data: CreateEncounterInput,
+): Promise<Encounter> {
+  return api.post(`/runs/${runId}/encounters`, data)
+}
+
+export function updateEncounter(
+  id: number,
+  data: UpdateEncounterInput,
+): Promise<Encounter> {
+  return api.patch(`/encounters/${id}`, data)
+}
+
+export function deleteEncounter(id: number): Promise<void> {
+  return api.del(`/encounters/${id}`)
+}