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-card.slint

@@ -1,4 +1,124 @@
-// AGENTS.md §7 Message Card.
+// AGENTS.md §7 message card + §34 MessageItem shape.
 //
-// Placeholder for Phase 2 C1 — real implementation lands in C2 alongside
-// `src/models.rs` (MessageItem / Priority) and the mock data feed.
+// `export enum Priority` and `export struct MessageItem` are the single source
+// of truth for the message domain; Slint generates matching Rust types under
+// the same names (used by `src/models.rs::mock_messages`). C2 wires them
+// into `ui/message-list.slint`; C3+ leaves the card body untouched but may
+// adjust the trailing "open-link" button copy.
+
+export enum Priority {
+    Low,
+    Normal,
+    High,
+    Max,
+}
+
+// Slint does not support `Option<String>`; the URL field is split into a
+// presence flag and the string itself. `has_click_url: false` + empty
+// `click_url` means "no link", exactly mirroring Rust's `Option<String>`.
+export struct MessageItem {
+    id: int,
+    app_id: int,
+    app_name: string,
+    title: string,
+    body: string,
+    priority: Priority,
+    timestamp_unix: int,
+    relative_time: string,
+    has_click_url: bool,
+    click_url: string,
+}
+
+// Color tokens are Dark defaults. C3 expands `MessageCard` callers to forward
+// the MainWindow palette when light/system themes land.
+export component MessageCard inherits Rectangle {
+    in property <MessageItem> item;
+    in property <bool> selected: false;
+
+    in property <color> bg-color: #28292d;
+    in property <color> bg-selected-color: #2f3035;
+    in property <color> card-border: #3a3b40;
+    in property <color> text-primary-color: #e6e6e6;
+    in property <color> text-secondary-color: #a0a0a0;
+    in property <color> text-muted-color: #6a6a6a;
+    in property <color> priority-low-color: #6a6a6a;
+    in property <color> priority-normal-color: #4ea1ff;
+    in property <color> priority-high-color: #ff9800;
+    in property <color> priority-max-color: #f44336;
+    in property <color> accent-color: #4ea1ff;
+
+    callback clicked();
+
+    width: 100%;
+    height: 76px;
+    border-radius: 8px;
+    background: selected ? bg-selected-color : bg-color;
+    border-color: card-border;
+    border-width: 1px;
+
+    HorizontalLayout {
+        padding: 16px;
+        spacing: 12px;
+        alignment: center;
+
+        // §7 priority indicator — small colored dot, NOT a saturated card.
+        Rectangle {
+            width: 8px;
+            height: 8px;
+            border-radius: 4px;
+            background: item.priority == Priority.Low    ? priority-low-color :
+                       item.priority == Priority.Normal ? priority-normal-color :
+                       item.priority == Priority.High   ? priority-high-color :
+                                                          priority-max-color;
+        }
+
+        VerticalLayout {
+            horizontal-stretch: 1;
+            spacing: 2px;
+
+            Text {
+                text: item.title != "" ? item.title : item.body;
+                color: text-primary-color;
+                font-size: 14px;
+                font-weight: 600;
+                overflow: elide;
+                horizontal-alignment: left;
+            }
+
+            Text {
+                text: item.app_name + " · " + item.body;
+                color: text-secondary-color;
+                font-size: 12px;
+                overflow: elide;
+                horizontal-alignment: left;
+            }
+        }
+
+        // Right column — relative time + optional link icon.
+        VerticalLayout {
+            spacing: 4px;
+            alignment: center;
+
+            Text {
+                text: item.relative_time;
+                color: text-muted-color;
+                font-size: 11px;
+                horizontal-alignment: right;
+            }
+
+            // Tiny chevron glyph to hint at clickability; only when a URL is present.
+            if item.has_click_url: Text {
+                text: "↗";
+                color: accent-color;
+                font-size: 12px;
+                horizontal-alignment: right;
+            }
+        }
+    }
+
+    TouchArea {
+        clicked => {
+            root.clicked();
+        }
+    }
+}