feat(config): unify common config snippets persistence across all apps

- Add unified `common_config_snippets` structure to MultiAppConfig
- Implement `get_common_config_snippet` and `set_common_config_snippet` commands
- Replace localStorage with config.json persistence for Codex and Gemini
- Auto-migrate legacy `claude_common_config_snippet` to new unified structure
- Deprecate individual API methods in favor of unified interface
- Add automatic migration from localStorage on first load

BREAKING CHANGE: Common config snippets now stored in unified `common_config_snippets` object instead of separate fields

src-tauri/src/lib.rs

@@ -517,6 +517,8 @@ pub fn run() {
             commands::open_app_config_folder,
             commands::get_claude_common_config_snippet,
             commands::set_claude_common_config_snippet,
+            commands::get_common_config_snippet,
+            commands::set_common_config_snippet,
             commands::read_live_provider_settings,
             commands::get_settings,
             commands::save_settings,