feat(plugin): 侧栏全页 AppPage、多 nav_key 联邦加载与 sidebar_nav 缓存

- 新增路由 plugin-app 与壳页，按 nav_key 尝试 AppPage{Pascal}/AppPage/Page
- DefaultLayout 与 appcenter 合并插件侧栏项；plugin/sidebar_nav 经 Pinia 去重缓存
- 工具 pluginSidebarNav、联邦 loader 与文档/示例更新；登出时清空侧栏缓存

Made-with: Cursor

examples/plugin-component/src/components/AppPage.vue

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+<script setup lang="ts">
+/**
+ * 侧栏全页：在主应用 #/plugin-app/:pluginId/:navKey 中渲染，占据主内容区。
+ * 需在插件后端实现 get_sidebar_nav 才会出现在侧栏。
+ */
+const props = defineProps({
+  api: {
+    type: Object,
+    default: () => ({}),
+  },
+  navKey: {
+    type: String,
+    default: 'main',
+  },
+  pluginId: {
+    type: String,
+    default: '',
+  },
+})
+
+const emit = defineEmits(['action'])
+</script>
+
+<template>
+  <div class="plugin-app-page pa-4">
+    <div class="text-h6 mb-2">AppPage（侧栏全页）</div>
+    <div class="text-body-2 text-medium-emphasis mb-4">
+      pluginId: {{ pluginId }} · navKey: {{ navKey }}
+    </div>
+    <v-btn size="small" variant="tonal" @click="emit('action')">action</v-btn>
+  </div>
+</template>

examples/plugin-component/src/components/AppPageSettings.vue

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+<script setup lang="ts">
+/**
+ * 示例：nav_key=settings 时主应用会优先加载 AppPageSettings，再回退 AppPage。
+ */
+const props = defineProps({
+  api: { type: Object, default: () => ({}) },
+  navKey: { type: String, default: 'settings' },
+  pluginId: { type: String, default: '' },
+})
+</script>
+
+<template>
+  <div class="pa-4">
+    <div class="text-subtitle-1">Settings 子界面（AppPageSettings）</div>
+    <div class="text-caption text-medium-emphasis">navKey={{ navKey }} · pluginId={{ pluginId }}</div>
+  </div>
+</template>