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/empty-state.slint

@@ -1,5 +1,75 @@
 // AGENTS.md §9 empty state.
 //
-// Placeholder for Phase 2 C1 — actual ✦ glyph + "You're all caught up"
-// panel lands in C2 when message-list integration knows whether the
-// backing model is empty.
+// Uses Slint `Path` to draw the ✦ glyph — no external assets. C3 may wire the
+// "Test Notification" button to a Phase 6 implementation that fires a real
+// Windows toast; C2 just invokes the callback so the click path is verifiable.
+
+export component EmptyState inherits Rectangle {
+    callback test-notification();
+
+    in property <color> glyph-color: #4ea1ff;
+    in property <color> text-primary-color: #e6e6e6;
+    in property <color> text-muted-color: #6a6a6a;
+    in property <color> button-bg: #2f3035;
+    in property <color> button-border: #3a3b40;
+    in property <color> button-text: #e6e6e6;
+
+    background: transparent;
+
+    VerticalLayout {
+        alignment: center;
+        spacing: 16px;
+        padding: 32px;
+
+        // §9 ✦ glyph — 4-point star drawn with Path.
+        Path {
+            width: 48px;
+            height: 48px;
+            fill: glyph-color;
+            commands: "M 24 0 L 28 20 L 48 24 L 28 28 L 24 48 L 20 28 L 0 24 L 20 20 Z";
+        }
+
+        Text {
+            text: "You're all caught up";
+            color: text-primary-color;
+            font-size: 18px;
+            font-weight: 600;
+            horizontal-alignment: center;
+        }
+
+        Text {
+            text: "New Gotify messages will\nappear here automatically.";
+            color: text-muted-color;
+            font-size: 13px;
+            horizontal-alignment: center;
+            wrap: word-wrap;
+        }
+
+        Rectangle { height: 8px; }
+
+        // "Test Notification" button — same visual language as NavItem.
+        Rectangle {
+            background: button-bg;
+            border-color: button-border;
+            border-width: 1px;
+            border-radius: 6px;
+            height: 36px;
+            width: 160px;
+            horizontal-stretch: 0;
+
+            Text {
+                x: (parent.width - self.width) / 2;
+                y: (parent.height - self.height) / 2;
+                text: "Test Notification";
+                color: button-text;
+                font-size: 13px;
+            }
+
+            TouchArea {
+                clicked => {
+                    root.test-notification();
+                }
+            }
+        }
+    }
+}