feat(app): add authenticated app shell with top bar and bottom navigation (#6)

### 概述 / Summary
实现认证后的主应用壳层（`MainAppShell`），包含胶囊顶栏（`ABoxTopPillBar`）与 Material 3 底部导航栏（`ABoxNavigationBar`），并为团队其他成员预留标准 Composable 插槽。

### 核心变更 / Changes
1. **设计系统 (`:android:core:designsystem`)**：
   - 新增 `ABoxTopPillBar`：展示提供方/空间信息、快捷 `+` 操作与用户头像徽标。
   - 新增 `ABoxUserAvatar`：支持用户名首字符与主题强调轮廓。
   - 新增 `ABoxNavigationBar` & `ABoxNavigationBarItem`：M3 胶囊指示器底部导航栏封装。
2. **应用壳层 (`:android:app`)**：
   - 新增 `MainTab`（`DEVICES`, `SCENES`, `ACTIVITY`），严格对齐需求与分工术语。
   - 新增 `MainAppShell`：承载顶栏、底栏、Tab 平滑切换，并开放 `devicesSlot`、`scenesSlot`、`activitySlot`。
   - 新增 `AccountProviderBottomSheet`：点击头像展示当前会话/提供方信息、支持切换提供方与退出登录。
3. **多语言与测试**：
   - 补充 `values` 与 `values-zh-rCN` 双语资源。
   - 新增 `MainTabTest` 单元测试并通过全量 `check` 与 `test`。

### 分工保护 / Boundaries
- 仅提供顶栏、底栏与主页面容器骨架；
- 设备列表、情景编排与设备控制详情的具体 UI 保持为插槽，供 alZerNest、tzh 和 Aurum 后续无缝填充。

Reviewed-on: #6
Co-authored-by: KremeCN <3265494849@qq.com>

android/app/src/main/res/values/strings.xml

@@ -8,7 +8,6 @@
     <string name="provider_local_description">Explore the app with simulated local devices and no network connection.</string>
     <string name="login_title">Sign in to %1$s</string>
     <string name="login_provider_context">Selected provider: %1$s</string>
-    <string name="login_back_to_providers">Back to providers</string>
     <string name="login_username">Username</string>
     <string name="login_password">Password</string>
     <string name="login_remember_username">Remember username</string>
@@ -24,4 +23,23 @@
     <string name="action_switch_provider">Switch provider</string>
     <string name="authenticated_title">Connected to %1$s</string>
     <string name="authenticated_description">The authenticated route is ready. Device navigation will be added by its module owner.</string>
+
+    <!-- Authenticated shell -->
+    <string name="tab_devices">Devices</string>
+    <string name="tab_scenes">Scenes</string>
+    <string name="tab_activity">Activity</string>
+    <string name="top_bar_prompt_format">Ask "%1$s"&#8230;</string>
+    <string name="action_add">Add</string>
+    <string name="action_account">Account &amp; Provider</string>
+    <string name="account_sheet_title">Account &amp; Provider</string>
+    <string name="account_sheet_signed_in_as">Signed in as</string>
+    <string name="account_sheet_provider">Active Provider</string>
+    <string name="account_sheet_logout">Sign Out</string>
+    <string name="account_sheet_close">Close</string>
+    <string name="devices_placeholder_title">Device List</string>
+    <string name="devices_placeholder_description">Device list and control cards slot for alZerNest.</string>
+    <string name="scenes_placeholder_title">Scene Mode</string>
+    <string name="scenes_placeholder_description">Scene actions and execution slot for tzh.</string>
+    <string name="activity_placeholder_title">Activity Log</string>
+    <string name="activity_placeholder_description">No recorded events yet.</string>
 </resources>