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

ui/server-card.slint

@@ -1,6 +1,129 @@
 // AGENTS.md §21 multi-server card.
 //
-// Placeholder for Phase 2 C1. The Settings → Connection section uses these
-// to list/edit each configured server (name / URL / token / enabled).
-// Real data wiring lands in Phase 5 (Storage) when `AppConfig.servers` is
-// persisted.
+// Phase 2 C3 is a visual + structural shell: one placeholder row so the
+// Settings page has the right rhythm. The Settings `Connection` section
+// hosts a `Repeater` over these once `AppConfig.servers` lands in Phase 5.
+
+export component ServerCard inherits Rectangle {
+    in property <string> server-name: "(unnamed server)";
+    in property <string> server-url: "";
+    in property <bool> enabled: true;
+    in property <bool> is-current: false;
+
+    callback toggled();
+    callback edit-clicked();
+    callback remove-clicked();
+
+    in property <color> bg-color: #28292d;
+    in property <color> card-border: #3a3b40;
+    in property <color> text-primary-color: #e6e6e6;
+    in property <color> text-secondary-color: #a0a0a0;
+    in property <color> accent-color: #4ea1ff;
+    in property <color> success-color: #4caf50;
+    in property <color> toggle-on: #4ea1ff;
+    in property <color> toggle-off: #6a6a6a;
+    in property <color> button-bg: #2f3035;
+
+    width: 100%;
+    height: 64px;
+    border-radius: 8px;
+    background: bg-color;
+    border-color: card-border;
+    border-width: 1px;
+
+    HorizontalLayout {
+        padding-top: 12px;
+        padding-bottom: 12px;
+        padding-left: 16px;
+        padding-right: 16px;
+        spacing: 12px;
+        alignment: center;
+
+        // On/off pill toggle (visual only in C3 — Phase 5 makes it mutate state).
+        Rectangle {
+            width: 36px;
+            height: 20px;
+            border-radius: 10px;
+            background: enabled ? toggle-on : toggle-off;
+            y: parent.height / 2 - self.height / 2;
+
+            Rectangle {
+                width: 16px;
+                height: 16px;
+                border-radius: 8px;
+                background: #ffffff;
+                x: enabled ? parent.width - self.width - 2px : 2px;
+                y: (parent.height - self.height) / 2;
+            }
+            TouchArea {
+                clicked => {
+                    root.toggled();
+                }
+            }
+        }
+
+        VerticalLayout {
+            horizontal-stretch: 1;
+            spacing: 2px;
+
+            HorizontalLayout {
+                spacing: 8px;
+                Text {
+                    text: server-name;
+                    color: text-primary-color;
+                    font-size: 13px;
+                    font-weight: 600;
+                }
+                if is-current: Text {
+                    text: "• current";
+                    color: accent-color;
+                    font-size: 11px;
+                }
+            }
+            Text {
+                text: server-url;
+                color: text-secondary-color;
+                font-size: 11px;
+                overflow: elide;
+            }
+        }
+
+        // Action buttons.
+        Rectangle {
+            width: 56px;
+            height: 28px;
+            background: button-bg;
+            border-radius: 6px;
+            Text {
+                x: (parent.width - self.width) / 2;
+                y: (parent.height - self.height) / 2;
+                text: "Edit";
+                color: text-primary-color;
+                font-size: 11px;
+            }
+            TouchArea {
+                clicked => {
+                    root.edit-clicked();
+                }
+            }
+        }
+        Rectangle {
+            width: 56px;
+            height: 28px;
+            background: button-bg;
+            border-radius: 6px;
+            Text {
+                x: (parent.width - self.width) / 2;
+                y: (parent.height - self.height) / 2;
+                text: "Remove";
+                color: text-secondary-color;
+                font-size: 11px;
+            }
+            TouchArea {
+                clicked => {
+                    root.remove-clicked();
+                }
+            }
+        }
+    }
+}