👷 ci: add multi-platform build workflow and refactor API structure

Add GitHub Actions workflow for building example app across all platforms
(Windows, macOS, Linux, Android, iOS) with automated releases.

Refactor core API modules for better organization:
- Restructure engine, error, runtime, and source modules
- Update FRB codegen for new API structure
- Remove legacy engine.dart file
- Update integration tests and example screens

BREAKING CHANGE: API module restructuring affects import paths

Author: iota9star

.github/workflows/build-all-platforms.yml

@@ -1,5 +1,19 @@
 # Changelog
 
+## 1.5.0
+
+* **BREAKING**: Simplified Engine API - Removed `JsAction`, `JsCallback`, `JsCallbackResult`, and `JsActionResult` enums
+* **BREAKING**: Replaced action-based messaging with direct async methods for better developer experience
+* **FEATURE**: New direct async methods on `JsEngineCore`: `eval()`, `evaluateModule()`, `declareNewModule()`, `declareNewModules()`, `clearNewModules()`, `getDeclaredModules()`, `isModuleDeclared()`, `call()`
+* **FEATURE**: Simplified bridge callback - now uses `Fn(JsValue) -> DartFnFuture<JsResult>` instead of complex enum-based callbacks
+* **FEATURE**: Added `initWithoutBridge()` method for engines that don't need Dart bridge communication
+* **IMPROVEMENT**: Reduced code complexity by removing the action queue and channel-based messaging
+* **IMPROVEMENT**: Better error handling with direct result returns instead of callback-based error propagation
+* **IMPROVEMENT**: Cleaner API surface - engine methods now return results directly
+* **INTERNAL**: Updated FRB codegen for new API structure
+* **INTERNAL**: Refactored internal state management using `Arc<RwLock<Option<...>>>` for bridge storage
+* **DOCS**: Updated workflow configuration for FJS example builds
+
 ## 1.4.0
 
 * **BREAKING**: Restructured API modules - removed deprecated `js` module, added `engine`, `error`, `runtime`, and `source` modules for better organization