feat(ui): overhaul frontend design system and layout

Introduce reusable UI components (AppShell, EntityCard, PageHeader)
Implement Pokemon-themed CSS variables and responsive grids
Refactor all views to adopt the new component structure

frontend/src/components/AppShell.vue

@@ -0,0 +1,50 @@
+<script setup lang="ts">
+import type { AuthUser } from '../services/api';
+import PokeBallMark from './PokeBallMark.vue';
+
+defineProps<{
+  currentUser: AuthUser | null;
+  navItems: Array<{ label: string; to: string }>;
+}>();
+
+defineEmits<{
+  logout: [];
+}>();
+</script>
+
+<template>
+  <div class="app-shell">
+    <header class="site-header">
+      <div class="container top-nav">
+        <RouterLink class="brand-lockup" to="/pokemon" aria-label="Pokopia Wiki">
+          <PokeBallMark size="42px" />
+          <span>
+            <span class="pokemon-word">Pokopia</span>
+            <span class="brand-subtitle">Community Wiki</span>
+          </span>
+        </RouterLink>
+
+        <nav class="nav-links" aria-label="主导航">
+          <RouterLink v-for="item in navItems" :key="item.to" :to="item.to">
+            {{ item.label }}
+          </RouterLink>
+        </nav>
+
+        <div class="auth-actions">
+          <template v-if="currentUser">
+            <span class="auth-user">{{ currentUser.displayName || currentUser.email }}</span>
+            <button class="ui-button ui-button--ghost ui-button--small" type="button" @click="$emit('logout')">退出</button>
+          </template>
+          <template v-else>
+            <RouterLink class="ui-button ui-button--ghost ui-button--small" to="/login">登录</RouterLink>
+            <RouterLink class="ui-button ui-button--primary ui-button--small" to="/register">注册</RouterLink>
+          </template>
+        </div>
+      </div>
+    </header>
+
+    <main class="page">
+      <slot></slot>
+    </main>
+  </div>
+</template>