feat: inline PDF viewer with pinch-to-zoom

- Add pdfrx package for native PDF rendering
- Tap PDF file card opens full-screen viewer inside the app
- Supports pinch-to-zoom and page navigation
- Other file types still open via iOS share sheet

	ios/Podfile.lock	diff | view | blame | history
	lib/widgets/message_bubble.dart	diff | view | blame | history
	lib/widgets/pdf_viewer.dart	diff | view | blame | history
	linux/flutter/generated_plugins.cmake	diff | view | blame | history
	pubspec.lock	diff | view | blame | history
	pubspec.yaml	diff | view | blame | history
	windows/flutter/generated_plugins.cmake	diff | view | blame | history