feat(windows): Phase 6 C3 — 系统托盘 + URL 打开

- src/windows/url.rs: open() 用 url::Url parse 校验 + scheme 严格
  限制（只 http/https，拒绝 file:// / javascript: / data: / ftp /
  自定义 scheme）；通过 ShellExecuteW 调默认浏览器打开；§13
  / §38 安全约束
- src/windows/tray.rs:
  · RegisterClassExW 注册隐藏 helper window class
  · CreateWindowExW(WS_EX_TOOLWINDOW, 0×0) 创建不显示的
    message-only helper window
  · Shell_NotifyIconW NIM_ADD 注册托盘图标（默认 NULL icon，
    Phase 7 polish 再换 .ico）
  · dedicated thread 跑 GetMessageW / DispatchMessageW 标准
    message loop
  · WM_USER+1 uCallbackMessage 处理 tray 鼠标事件
    · WM_LBUTTONDBLCLK = Open callback
    · WM_RBUTTONUP = CreatePopupMenu + TrackPopupMenuEx 显示
      Open / Quit 菜单
  · WM_COMMAND 处理菜单 item IDM_OPEN / IDM_QUIT
  · Drop 时 NIM_DELETE + DestroyWindow + thread.join() 干净退出
- src/lib.rs:
  · #[cfg(windows)] 启动时建 TrayIcon，绑 on_open（show window）
    / on_quit（std::process::exit(0)）；失败时只 warn 不 abort
    （托盘是便利功能，不致命）
  · 引入 url::open；Phase 7 polish 时接到 message-card 点击
- src/windows/mod.rs: pub use tray + url
- Cargo.toml: windows-sys features 加 Win32_Graphics_Gdi
  （WNDCLASSEXW 依赖）

避坑：
- LRESULT / WPARAM / LPARAM 在 windows-sys 0.59 是 type
  alias，不是 struct — 不能 LRESULT(0) 构造；直接 0 返回
  隐式 cast
- HMENU 是 *mut c_void；用 std::ptr::null_mut()（不是 null()）
- NOTIFYICONDATAW_0 在 0.59 没 derive Default；用
  unsafe { std::mem::zeroed() }
- std::mem::GUID::zeroed() 不存在；用 windows_sys::core::GUID
  显式构造
- NIM_VERSION 常量 windows-sys 0.59 没 export；用字面 4u32
  （NIM_VERSION = 4 in Win32 header）
- HWND (raw pointer) 不 Send；跨线程传递时转 usize
- Box<dyn Fn> 不能 derive Clone；用 Box + 唯一 move 到
  TrayShared 不需要 Clone
- on_quit closure 不能捕获 Rc<RefCell<AppState>>（不是 Sync）
  → 简化：on_quit 直接 std::process::exit(0)；之前 close
  已经 save_state，丢失窗口只是 size 增量
- WNDCLASSEXW + RegisterClassExW 在 Win32_Graphics_Gdi
  feature 后（hbrBackground 用 HBRUSH = GDI 类型）

验证：cargo build / build --release 都 exit 0，0 warning
- cargo test: 19 passed (新增 4 个 url rejection 测试 +
  之前的 15 个); 0 failed
- debug   35.66 MB, subsystem = WINDOWS_CUI
- release  9.79 MB, subsystem = WINDOWS_GUI
- 托盘只能手测（CI 不能断言 WM_USER 消息）；自动测试覆盖
  URL 校验逻辑的拒绝路径

src/lib.rs

@@ -39,6 +39,11 @@ use slint::{ModelRc, VecModel};
 
 use storage::AppState;
 
+#[cfg(windows)]
+use crate::windows::tray::{TrayCallbacks, TrayIcon};
+#[cfg(windows)]
+use crate::windows::url as url_opener;
+
 /// Map the persisted theme string (`state.json::last_theme`) back into
 /// the Slint `AppearanceMode` enum. Unknown values fall back to Dark so
 /// a corrupted config file can't strand the UI on Light with no way to
@@ -229,6 +234,45 @@ pub fn run() -> error::AppResult<()> {
         tracing::debug!(idx, "message card clicked");
     });
 
+    // ── Phase 6 C3: tray icon (Windows only) ──────────────────────
+    // The tray owns a hidden helper window and a dedicated message-
+    // pump thread. `Open` shows the window; `Quit` exits the
+    // process. State is not saved from the quit path on purpose: any
+    // `close-to-tray` close already wrote state.json, and the tray
+    // only runs after the main loop started, so a window-resize that
+    // happened *between* the last close and the quit click is the
+    // only data at risk (Phase 7 polish will route this through
+    // a proper invoke_from_event_loop if it matters).
+    #[cfg(windows)]
+    let _tray = {
+        let ui_handle_for_open = ui.as_weak();
+        TrayIcon::new(TrayCallbacks {
+            on_open: Box::new(move || {
+                if let Some(ui) = ui_handle_for_open.upgrade() {
+                    let _ = ui.window().show();
+                    tracing::info!("tray: Open → window shown");
+                }
+            }),
+            on_quit: Box::new(|| {
+                tracing::info!("tray: Quit → exit");
+                std::process::exit(0);
+            }),
+        })
+        .map_err(|e| {
+            tracing::warn!(error = %e, "tray: failed to install, continuing without it");
+        })
+        .ok()
+    };
+    #[cfg(windows)]
+    let _tray = _tray; // hold for the rest of run()
+
+    // ── Phase 6 C3: URL opener (Windows only) — already wired; the
+    // Gotify `click_url` field is reserved in `models.rs` (Phase 3)
+    // and is opened via this helper when Phase 7 polish wires a
+    // message-card click on `has_click_url`. ──
+    #[cfg(windows)]
+    let _ = url_opener::open; // keep the import alive for Phase 7
+
     // ── Phase 3 C2: Settings → Connection → Test button ───────────
     ui.on_test_connection_requested({
         let ui_handle = ui.as_weak();