Add "Report Bug" action to About screen, pre-seeded with app version and platform

[MrSurly]

Summary

The About screen/dialog (lib/widgets/app_menu_button.dart, _showAbout, using showAboutDialog with PackageInfo.fromPlatform() for version) has no way to report a bug directly. Add one.

Desired behavior

• A "Report Bug" action in the About screen/dialog.
• Opens a new GitHub issue (or whatever the team's actual bug-tracking destination is) pre-filled with:
  • App version (PackageInfo.fromPlatform() -- already used for the version shown in About, same source)
  • Platform/OS (e.g. Android/iOS/Linux + OS version)
• Consider also linking to or bundling the existing debug log export (lib/screens/debug_log_screen.dart, exportLogs/_exportLogs) so a useful report doesn't require the user to separately discover that screen.

Implementation notes

• url_launcher is already a dependency (added for tappable URLs in chat, Make URLs in chat messages tappable #52) -- could pre-fill a GitHub "new issue" URL with a title/body query string as one approach, or open the device's mail client, depending on what destination the team wants.
• Version/platform values are cheap to obtain (package_info_plus already in use; Platform.operatingSystem/Platform.operatingSystemVersion from dart:io for the rest).