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

ui/app.slint

@@ -7,6 +7,9 @@
 
 import { AppearanceMode } from "./theme.slint";
 import { NavItem, SectionLabel } from "./components.slint";
+import { MessageItem } from "./message-card.slint";
+import { MessageList } from "./message-list.slint";
+import { EmptyState } from "./empty-state.slint";
 
 export enum SidebarSelection {
     All,
@@ -48,6 +51,18 @@ export component MainWindow inherits Window {
 
     in-out property <SidebarSelection> selection: SidebarSelection.All;
 
+    // Phase 2 C2: mock-driven message list + selection state. Set from Rust
+    // via `ui.set_messages(model)` in `run()`. The list / empty-state switch
+    // is purely driven by `messages.length` (Slint's `for` over an empty
+    // model is a no-op).
+    in-out property <[MessageItem]> messages;
+    in-out property <int> selected-message-index: -1;
+
+    // Hooks that Phase 6 (Windows Integration) wires to a real Windows
+    // toast. C2 logs them so the click path is observable end-to-end.
+    callback test-notification-clicked();
+    callback message-card-clicked(int);
+
     HorizontalLayout {
         // §7 Sidebar width: 220-240px.
         Rectangle {
@@ -133,18 +148,29 @@ export component MainWindow inherits Window {
 
             if root.selection == SidebarSelection.All: VerticalLayout {
                 padding: 24px;
-                spacing: 8px;
+                spacing: 12px;
 
                 Text {
-                    text: "Messages";
+                    text: messages.length == 0 ? "Messages" : "Messages · " + messages.length;
                     color: text-primary;
                     font-size: 18px;
                     font-weight: 700;
                 }
-                Text {
-                    text: "Message list and empty state land in Phase 2 C2.";
-                    color: text-muted;
-                    font-size: 12px;
+
+                if messages.length > 0: MessageList {
+                    messages: messages;
+                    selected-index: root.selected-message-index;
+                    height: parent.height - 48px;
+                    message-clicked(idx) => {
+                        root.selected-message-index = idx;
+                        root.message-card-clicked(idx);
+                    }
+                }
+
+                if messages.length == 0: EmptyState {
+                    test-notification => {
+                        root.test-notification-clicked();
+                    }
                 }
             }