Add main settings view and theme management functionality

Introduce a new `MainSettings` view for managing application settings, including language and theme selection. Implement theme management methods in the `setting` package to handle theme initialization, retrieval, and updates.

internal/controller/menu.go

@@ -0,0 +1,36 @@
+package controller
+
+import (
+	"git.kor-elf.net/kor-elf/gui-for-ffmpeg/internal/gui/view"
+)
+
+func (c *controller) actionSettingConvertor() {
+	c.settingConvertor(true)
+}
+
+func (c *controller) actionMainSettings() {
+	currentLang, _ := c.app.GetSetting().GetCurrentLangOrDefaultLang()
+	content := view.MainSettings(
+		currentLang,
+		c.app.GetSetting().GetLanguages(),
+
+		c.app.GetSetting().GetTheme(),
+		c.app.GetSetting().GetThemes(),
+
+		c.actionMainSettingsSave,
+		c.convertor,
+	)
+	c.window.SetContent(content)
+}
+
+func (c *controller) actionMainSettingsSave(setting *view.MainSettingForm) error {
+	err := c.app.GetSetting().SetLang(setting.Language)
+	if err != nil {
+		return err
+	}
+	c.app.GetSetting().SetTheme(setting.ThemeInfo)
+	c.initLayout()
+
+	c.convertor()
+	return nil
+}