refactor(api): simplify app type parameter handling to single required parameter

Replace the previous dual-parameter approach (app_type/app/appType) with a single required `app: String` parameter across all Tauri commands. This change:

- Introduces unified `parse_app()` helper replacing complex `resolve_app_type()` logic
- Updates all backend commands in config, mcp, and provider modules
- Aligns frontend API calls to use consistent `app` parameter naming
- Simplifies MSW test handlers by removing optional parameter handling

This improves API clarity and reduces parameter ambiguity while maintaining backward compatibility through error handling.

src/lib/api/settings.ts

@@ -31,11 +31,11 @@ export const settingsApi = {
   },
 
   async getConfigDir(appType: AppType): Promise<string> {
-    return await invoke("get_config_dir", { app_type: appType });
+    return await invoke("get_config_dir", { app: appType });
   },
 
   async openConfigFolder(appType: AppType): Promise<void> {
-    await invoke("open_config_folder", { app_type: appType });
+    await invoke("open_config_folder", { app: appType });
   },
 
   async selectConfigDirectory(defaultPath?: string): Promise<string | null> {