feat(ui): Phase 2 C3 — Settings 页 + Dialogs + 窗口状态保存

- src/storage.rs: AppState (last_window_width/height) + load/save
  atomic write 到 %APPDATA%/WindowsGotify/state.json（§18）
- ui/settings-page.slint: 5 分组（Connection / Notifications / Startup
  / Appearance / Advanced），Appearance 实时切主题（System/Light/Dark
  radio）；Advanced 折叠
- ui/dialogs.slint: AboutDialog + ConfirmDialog（§54）
- ui/server-card.slint: 单 server 行（name/url/enabled toggle/edit/remove）
- ui/app.slint: settings 页切换、About 导航、theme 联动
- src/lib.rs: load_state → 启动时应用窗口尺寸；ui.window()
  .on_close_requested 在 OS 关闭前 save_state；wire about/close
  callbacks
- src/error.rs: 加 AppError::Storage(String) 变体
- Cargo.toml: 加 serde + serde_json

避坑：
- Slint 1.x 同文件内 export component 必须在引用前定义（sub-components
  移到 SettingsPage 上面）
- Slint 不支持 padding 两值简写 (padding: 0 16px) → 用 padding-* 四属性
- Slint 不支持 Rectangle 内建 border-color 覆盖 → 命名 card-border
- Slint 不支持 alignment: right → 用 spacer + horizontal-stretch: 1
- 修了一处 .slint 文件被 PowerShell Set-Content GBK 损坏的问题（已
  通过 write 工具 + UTF-8 no BOM 重写并清理 ui/ 所有 .slint）

验证：cargo build / build --release 都 exit 0，0 warning
- debug   27.55 MB, subsystem = WINDOWS_CUI
- release  8.10 MB, subsystem = WINDOWS_GUI

src/lib.rs

@@ -3,19 +3,22 @@
 //! Phase 2 surface:
 //!   - [`error`]:   AppError / AppResult
 //!   - [`logging`]: tracing-subscriber init
-//!   - [`models`]:  MessageItem / Priority (re-exported from slint-generated
-//!                  types) + the mock data feed used until Phase 3 lands.
+//!   - [`models`]:  Mock message feed (Phase 3 replaces with live Gotify data)
+//!   - [`storage`]: `%APPDATA%/WindowsGotify/state.json` (Phase 2 C3)
 //!   - [`run`]:     builds the MainWindow and runs its event loop
-//!
-//! Larger subsystems (gotify / storage / windows integration) land in later
-//! phases per AGENTS.md §57.
 
 pub mod error;
 pub mod logging;
 pub mod models;
+pub mod storage;
 
 slint::include_modules!();
 
+use std::cell::RefCell;
+use std::rc::Rc;
+
+use storage::AppState;
+
 /// Build the MainWindow and run its event loop until the window is closed.
 ///
 /// In release builds the Windows subsystem is set to `windows` via the
@@ -28,12 +31,20 @@ pub fn run() -> error::AppResult<()> {
 
     let ui = MainWindow::new().map_err(|e| error::AppError::Ui(e.to_string()))?;
 
-    // Phase 2 C1: dark by default. C3 wires this to the Appearance section
-    // of the Settings page and to `state.json` so the choice persists.
-    ui.set_theme(AppearanceMode::Dark);
+    // Phase 2 C3: load persisted state and apply it before `run()`.
+    let initial_state = storage::load_state();
+    tracing::info!(
+        width = initial_state.last_window_width,
+        height = initial_state.last_window_height,
+        "loaded state.json"
+    );
+    ui.set_theme(AppearanceMode::Dark); // C4 reads theme from state.json.
+    ui.window().set_size(slint::WindowSize::Logical(slint::LogicalSize::new(
+        initial_state.last_window_width.max(0) as f32,
+        initial_state.last_window_height.max(0) as f32,
+    )));
 
-    // Phase 2 C2: hook up the two top-level callbacks. C6/Phase 6 swap these
-    // for real Windows toast firing.
+    // Phase 2 C2 callbacks.
     ui.on_test_notification_clicked(|| {
         tracing::info!("empty-state: Test Notification clicked (stub)");
     });
@@ -41,10 +52,49 @@ pub fn run() -> error::AppResult<()> {
         tracing::debug!(idx, "message card clicked");
     });
 
-    // Phase 2 C2: feed mock data into the message list. Phase 3 swaps this
-    // for an HTTP catch-up + WebSocket live stream.
+    // Phase 2 C3 callbacks.
+    let state_for_save: Rc<RefCell<AppState>> = Rc::new(RefCell::new(initial_state));
+    ui.on_about_clicked({
+        let ui_handle = ui.as_weak();
+        move || {
+            tracing::info!("about-clicked");
+            if let Some(ui) = ui_handle.upgrade() {
+                ui.set_selection(SidebarSelection::About);
+            }
+        }
+    });
+
+    // Phase 2 C2 message feed (mock).
     ui.set_messages(models::mock_messages_model());
 
+    // Hook a real close-requested callback on the Slint window so we save
+    // state right before the window is destroyed. Slint 1.18's `Window` has
+    // a `close-requested` callback that fires on OS-close.
+    ui.window().on_close_requested({
+        let state_for_save = state_for_save.clone();
+        let ui_handle = ui.as_weak();
+        move || {
+            // Capture final size from the live window before saving.
+            if let Some(ui) = ui_handle.upgrade() {
+                let size = ui.window().size();
+                let mut state = state_for_save.borrow_mut();
+                state.last_window_width = size.width as i32;
+                state.last_window_height = size.height as i32;
+            }
+            if let Err(e) = storage::save_state(&state_for_save.borrow()) {
+                tracing::warn!(error = %e, "on close: failed to save state.json");
+            } else {
+                tracing::info!(
+                    width = state_for_save.borrow().last_window_width,
+                    height = state_for_save.borrow().last_window_height,
+                    "saved state.json on close"
+                );
+            }
+            // Allow the window to hide (default CloseRequestResponse behaviour).
+            slint::CloseRequestResponse::HideWindow
+        }
+    });
+
     ui.run()
         .map_err(|e| error::AppError::Ui(e.to_string()))
 }