feat: integrate language switcher into settings with modern segment control UI

- Move language switcher from header to settings modal for better organization
- Implement modern segment control UI instead of radio buttons for language selection
- Add language preference persistence in localStorage and backend settings
- Support instant language preview with cancel/revert functionality
- Remove standalone LanguageSwitcher component
- Improve initial language detection logic (localStorage -> browser -> default)
- Add proper i18n keys for language settings UI text

src-tauri/src/settings.rs

@@ -15,6 +15,8 @@ pub struct AppSettings {
     pub claude_config_dir: Option<String>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub codex_config_dir: Option<String>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub language: Option<String>,
 }
 
 fn default_show_in_tray() -> bool {
@@ -32,6 +34,7 @@ impl Default for AppSettings {
             minimize_to_tray_on_close: true,
             claude_config_dir: None,
             codex_config_dir: None,
+            language: None,
         }
     }
 }
@@ -55,6 +58,13 @@ impl AppSettings {
             .map(|s| s.trim())
             .filter(|s| !s.is_empty())
             .map(|s| s.to_string());
+
+        self.language = self
+            .language
+            .as_ref()
+            .map(|s| s.trim())
+            .filter(|s| matches!(*s, "en" | "zh"))
+            .map(|s| s.to_string());
     }
 
     pub fn load() -> Self {