Flutter integration for log_trail.

• recordFlutterError: an extension on any Logger (the root or a module's child logger) that records Flutter errors (FlutterErrorDetails): FlutterError.onError = logger.recordFlutterError. They become ordinary entries whose LogError.report carries the original details, so every sink and sink wrapper receives them; Flutter-aware sinks read the details from there. recordPlatformError fits PlatformDispatcher.instance.onError, which expects a handler returning bool.
• flutterConsoleSink(): the console sink from log_trail preset to print through debugPrint in FlutterConsoleFormat, which renders Flutter errors with the full FlutterErrorDetails diagnostics, in the same shape as FlutterError.dumpErrorToConsole, without ANSI colors by default: debugPrint output reaches the IDE through os_log/syslog on iOS, which turn the escape codes into text. The format is a LogEntryFormat and can be used with any ConsoleSink or replaced.
• LoggingNavigatorObserver: a NavigatorObserver that logs route changes. It depends only on package:flutter/widgets.dart, not on Material.
• LogViewerPage: the in-app log viewer on top of a LogReader (any LogStore) (package:log_trail_flutter/widgets.dart, which also re-exports package:log_trail/storage.dart). Its texts are English by default and can be translated with LogViewerStrings.

The example app wires all of this together: the root logger created first, error hooks, the navigator observer, a few produced entries and the viewer with export to the clipboard (flutter run in example/).

See the log_trail README for the motivation and the overall design.