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/usage.ts

@@ -6,7 +6,7 @@ export const usageApi = {
   async query(providerId: string, appType: AppType): Promise<UsageResult> {
     return await invoke("query_provider_usage", {
       provider_id: providerId,
-      app_type: appType,
+      app: appType,
     });
   },
 };