feat(tray): add Gemini support to system tray menu (#209)

Refactor tray menu system to support three applications (Claude/Codex/Gemini):
- Introduce generic TrayAppSection structure and TRAY_SECTIONS array
- Implement append_provider_section and handle_provider_tray_event helper functions
- Enhance Gemini provider service with .env config read/write support
- Implement Gemini LiveSnapshot for atomic operations and rollback
- Update README documentation to reflect Gemini tray quick switching feature

src-tauri/tests/provider_service.rs

@@ -220,9 +220,7 @@ fn packycode_partner_meta_triggers_security_flag_even_without_keywords() {
             partner_promotion_key: Some("packycode".to_string()),
             ..ProviderMeta::default()
         });
-        manager
-            .providers
-            .insert("packy-meta".to_string(), provider);
+        manager.providers.insert("packy-meta".to_string(), provider);
     }
 
     let state = AppState {