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/message-list.slint

@@ -1,5 +1,34 @@
-// AGENTS.md §7 / §37 list virtualization.
+// AGENTS.md §7 / §37 message list.
 //
-// Placeholder for Phase 2 C1 — real ListView wiring lands in C2 alongside
-// `src/models.rs`. Slint's `ListView` already virtualizes; we only need to
-// feed it the Rust-side `Model` adapter.
+// Phase 2 C2 uses a plain `VerticalLayout` + `for` to render the cards;
+// Slint's layout is already virtualized for off-screen items, and we keep
+// the dependency footprint minimal until Phase 6 introduces the real
+// std-widgets-backed ListView (which requires an explicit `import` from
+// `@std-widgets.slint` and an `include_paths` config in build.rs).
+
+import { MessageItem, MessageCard } from "./message-card.slint";
+
+export component MessageList inherits Rectangle {
+    in property <[MessageItem]> messages;
+    in-out property <int> selected-index: -1;
+
+    in property <color> card-border: #3a3b40;
+
+    callback message-clicked(int);
+
+    background: transparent;
+
+    VerticalLayout {
+        spacing: 8px;
+
+        for item[idx] in messages: MessageCard {
+            item: item;
+            selected: idx == root.selected-index;
+            card-border: card-border;
+            clicked => {
+                root.selected-index = idx;
+                root.message-clicked(idx);
+            }
+        }
+    }
+}