feat(ui): Phase 2 C2 — Mock 消息模型 + Message list + Empty state

- ui/message-card.slint: export enum Priority + struct MessageItem +
  MessageCard 组件（§7 优先级圆点 + 标题 + 应用·预览 + 相对时间 +
  URL 指示）
- ui/message-list.slint: VerticalLayout + for 渲染 MessageCard
  （避开 std-widgets 的 include_paths 配置需求）
- ui/empty-state.slint: §9 ✦ glyph（Path 自绘）+ "You're all caught up"
  + Test Notification 按钮
- ui/app.slint: messages.length 切换 list / empty-state；点击回
  调到 MainWindow 顶层 callback
- src/models.rs: mock_messages() 生成 12 条覆盖 4 个 priority 的
  假消息；format_relative() 输出 "2 min ago" / Yesterday 等
- src/lib.rs: ui.set_messages(mock_messages_model()) 装载；wire
  on_test_notification_clicked / on_message_card_clicked

避坑：
- Slint 1.18 不允许覆盖 Rectangle 内建 border-color，命名用 card-border
- ModelRc::from 需要 Rc<VecModel<T>> 不是裸 VecModel

验证：cargo build / build --release 都 exit 0，0 warning
- debug   26.55 MB, subsystem = WINDOWS_CUI
- release  7.94 MB, subsystem = WINDOWS_GUI

src/lib.rs

@@ -1,17 +1,18 @@
 //! Windows Gotify Client (Rust) — core library.
 //!
-//! Phase 2 C1 only:
+//! Phase 2 surface:
 //!   - [`error`]:   AppError / AppResult
 //!   - [`logging`]: tracing-subscriber init
+//!   - [`models`]:  MessageItem / Priority (re-exported from slint-generated
+//!                  types) + the mock data feed used until Phase 3 lands.
 //!   - [`run`]:     builds the MainWindow and runs its event loop
 //!
-//! Larger subsystems (gotify / storage / windows integration / mock data)
-//! land in later commits per AGENTS.md §57.
+//! Larger subsystems (gotify / storage / windows integration) land in later
+//! phases per AGENTS.md §57.
 
 pub mod error;
 pub mod logging;
-
-use slint::ComponentHandle;
+pub mod models;
 
 slint::include_modules!();
 
@@ -31,6 +32,19 @@ pub fn run() -> error::AppResult<()> {
     // of the Settings page and to `state.json` so the choice persists.
     ui.set_theme(AppearanceMode::Dark);
 
+    // Phase 2 C2: hook up the two top-level callbacks. C6/Phase 6 swap these
+    // for real Windows toast firing.
+    ui.on_test_notification_clicked(|| {
+        tracing::info!("empty-state: Test Notification clicked (stub)");
+    });
+    ui.on_message_card_clicked(|idx| {
+        tracing::debug!(idx, "message card clicked");
+    });
+
+    // Phase 2 C2: feed mock data into the message list. Phase 3 swaps this
+    // for an HTTP catch-up + WebSocket live stream.
+    ui.set_messages(models::mock_messages_model());
+
     ui.run()
         .map_err(|e| error::AppError::Ui(e.to_string()))
 }