feat(ui): 主题切换 + 语言持久化 + About 对话框

- 新增 light/dark 主题调色板与 Windows 注册表系统主题探测
- 偏好设置对话框：主题三选一 + 语言下拉(en/zh-CN/de)，即时生效
- 持久化 Language/Theme/AutoLockMinutes/MinimizeToTray 至 YAML 顶层
- 启动时按 config 中保存的语言初始化 i18n
- About 对话框展示 version/runtime/项目地址/许可证
- 全部硬编码颜色迁移到 themePalette，为深色模式做准备

internal/config/model.go

@@ -21,9 +21,17 @@ type Entry struct {
 // default; if Encrypted is true, EncryptedBlob holds a WAGO1 base64 ciphertext
 // produced by internal/crypto.EncryptModern and Entries is empty on disk.
 type Config struct {
-	Version       int     `yaml:"version" json:"version"`
-	Language      string  `yaml:"language,omitempty" json:"language,omitempty"`
-	Encrypted     bool    `yaml:"encrypted" json:"encrypted"`
-	EncryptedBlob string  `yaml:"encrypted_blob,omitempty" json:"encrypted_blob,omitempty"`
-	Entries       []Entry `yaml:"entries,omitempty" json:"entries,omitempty"`
+	Version  int    `yaml:"version" json:"version"`
+	Language string `yaml:"language,omitempty" json:"language,omitempty"`
+	// Theme is one of "", "light", "dark", "system" (empty = system).
+	Theme string `yaml:"theme,omitempty" json:"theme,omitempty"`
+	// AutoLockMinutes locks the UI (hides codes, requires password) after
+	// this many minutes of inactivity. 0 disables auto-lock.
+	AutoLockMinutes int `yaml:"auto_lock_minutes,omitempty" json:"auto_lock_minutes,omitempty"`
+	// MinimizeToTray sends the window to the system tray instead of
+	// exiting when the close button is pressed.
+	MinimizeToTray bool    `yaml:"minimize_to_tray,omitempty" json:"minimize_to_tray,omitempty"`
+	Encrypted      bool    `yaml:"encrypted" json:"encrypted"`
+	EncryptedBlob  string  `yaml:"encrypted_blob,omitempty" json:"encrypted_blob,omitempty"`
+	Entries        []Entry `yaml:"entries,omitempty" json:"entries,omitempty"`
 }