refactor(settings): move device-level settings to local file storage

Separate device-level settings from database to support cloud sync scenarios
where multiple devices share the same database but need independent settings.

Changes:
- Remove database storage logic (bind_db, save_to_db, load_from_db)
- Remove legacy custom_endpoints_claude/codex fields (actual data in provider.meta)
- Add current_provider_claude/codex/gemini fields for device-level provider selection
- Add get_current_provider() and set_current_provider() helper functions
- Settings now stored only in ~/.cc-switch/settings.json

This ensures that when database is synced across devices, each device
maintains its own current provider selection independently.

src-tauri/src/lib.rs

@@ -580,7 +580,6 @@ pub fn run() {
                 }
             }
 
-            crate::settings::bind_db(db.clone());
             let app_state = AppState::new(db);
 
             // 检查是否需要首次导入（数据库为空）