feat(gotify): Phase 3 C2 — UI 接线 + Connection status + 真实数据

- src/config.rs: AppConfig::placeholder() 硬编码 NAS 地址 + 假 token
  （§20 token 永不日志；Phase 5 接持久化前占位）
- src/models.rs: gotify_messages_to_items 把 GotifyMessage 列表转
  Slint MessageItem[]；try_fetch_real_messages 一次完成 health →
  applications → messages(50)；失败 None 让调用方 fallback mock
- src/gotify/mod.rs: re-export ServerConfig
- src/lib.rs: run() 启动时调 try_fetch_real_messages，成功用真实数
  据 + Connected + 服务端 URL，失败 fallback mock + Disconnected；
  on_test_connection_requested 重新跑同一路径
- Cargo.toml: 加 chrono 0.4 (RFC3339 解析)
- ui/app.slint: is-connected (绿/灰 dot) + status-text (顶部副标)
- ui/settings-page.slint: Connection section 加 Test Connection 按钮
  （in-progress 时禁用显示 "Testing..."）

避坑：
- Gotify RFC3339 date 解析失败 silent fallback 到 unix=0（显示
  "just now" 而不是崩）
- Gotify priority 0..10 → Slint Priority 4 桶分桶：<=0 Low / <=3
  Normal / <=7 High / else Max
- ServerConfig 在 crate::gotify::model 子模块，mod.rs 必须 re-export

验证：cargo build / build --release 都 exit 0，0 warning
- cargo test --test gotify_api: 6 passed
- debug   33.36 MB, subsystem = WINDOWS_CUI
- release  9.43 MB, subsystem = WINDOWS_GUI（chrono +1.3 MB）

ui/settings-page.slint

@@ -137,6 +137,10 @@ export component SettingsPage inherits Rectangle {
     // Advanced (collapsed by default per §10)
     in-out property <bool> advanced-expanded: false;
 
+    // Connection (Phase 3 C2)
+    in-out property <bool> test-in-progress: false;
+    callback test-connection-requested();
+
     // Tokens
     in property <color> divider-color: #3a3b40;
     in property <color> hint-color: #6a6a6a;
@@ -151,6 +155,29 @@ export component SettingsPage inherits Rectangle {
         VerticalLayout {
             spacing: 8px;
             SectionLabel { label: "CONNECTION"; text-color: hint-color; }
+
+            // "Test Connection" button — Phase 3 C2 wires this to the
+            // real `GotifyApi::health` round-trip.
+            Rectangle {
+                width: 160px;
+                height: 32px;
+                background: #2f3035;
+                border-radius: 6px;
+                Text {
+                    x: (parent.width - self.width) / 2;
+                    y: (parent.height - self.height) / 2;
+                    text: root.test-in-progress ? "Testing..." : "Test Connection";
+                    color: #e6e6e6;
+                    font-size: 12px;
+                }
+                TouchArea {
+                    enabled: !root.test-in-progress;
+                    clicked => {
+                        root.test-connection-requested();
+                    }
+                }
+            }
+
             ServerCard {
                 server-name: "Home";
                 server-url: "https://gotify.example.com";