fix: resolve analyzer errors and bump to 2.0.0

[Matkurban]

Summary

This PR fixes all analyzer errors/warnings, modernizes the codebase for current Flutter/Dart, and bumps the package to 2.0.0.

Breaking change — the minimum required versions are raised to Dart ^3.13.0 and Flutter >=3.47.0, and the package now depends on package:material_ui instead of package:flutter/material.dart.

Changes

Bug fixes

• ObserverController: fix local variable shadowing errors (var controller = controller;) in _scrollToIndex, _handleScrollToIndexForFixedHeight and _handleScrollToIndex that prevented compilation under flutter_lints 6 (no_leading_underscores_for_local_identifiers).
• ObserverWidget: fix the same shadowing error for scopeContext in _checkTagChange.
• ChatScrollObserver: fix self-assignment of innerRefItemIndex, innerRefItemIndexAfterUpdate and innerRefItemLayoutOffset in standby.
• Guard BuildContext usage across async gaps with mounted checks (use_build_context_synchronously).

Tooling / lint

• Upgrade flutter_lints to ^6.0.0 in both the package and the example, and resolve every reported lint (super parameters, base class modifiers, null-aware elements, sort_child_properties_last, etc.).
• Format all Dart sources with the Dart 3.13 formatter so dart format --set-exit-if-changed passes in CI.
• Pin the example's any constraints to concrete versions.
• Replace the example's outdated counter smoke test with a home page smoke test.

Release

• Bump version to 2.0.0 and add the CHANGELOG entry.

Verification

Check	Result
flutter analyze (package)	No issues found
flutter analyze (example)	No issues found
dart format . -o none --set-exit-if-changed	0 changed
flutter test (package)	83 tests passed
flutter test (example)	1 test passed
flutter pub publish --dry-run	OK

Environment: Flutter 3.47.3 (stable) / Dart 3.13.3.

Notes for reviewers

• The deprecated APIs annotated with @Deprecated('It will be removed in version 2 ...') are intentionally kept in this PR to keep the change reviewable; they can be removed in a follow-up.
• Most of the 143 changed files are the mechanical material.dart → material_ui.dart import swap and formatter output; the behavioral fixes are concentrated in observer_controller.dart, observer_widget.dart and chat_scroll_observer.dart.

[LinXunFeng]

Thanks a lot for the effort! Since scrollview_observer doesn't use any Material widgets, #156 can be solved by simply importing package:flutter/widgets.dart instead of material.dart. That way we don't need a material_ui dependency or to raise the minimum SDK to Flutter 3.47, and it can ship as a patch release. So I'm going to close this PR in favor of that smaller change.