* Fix example pubspec SDK/lint constraints to match the package

example/pubspec.yaml declared sdk: ">=2.12.0-0 <3.0.0", which does not
overlap with the package's own >=3.0.0 <4.0.0. Since the example depends
on octo_image by path, no SDK the package supports satisfied the
example's own constraint. Align sdk, add the matching flutter lower
bound, and bump flutter_lints to ^4.0.0 to match the package's dev
dependency.

The flutter_lints bump surfaces four pre-existing use_super_parameters
findings in example/lib (Key? key : super(key: key)), which now fail
`flutter analyze` at the repo root since flutter_lints 1.0.4 predates
that lint. Convert those four constructors to super.key to keep analyze
green.

http stays as a dependency: example/lib/helpers/mock_image_provider.dart
imports it and is reachable from main_sets.dart, an alternate example
entry point not wired into main.dart.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Bump version to 2.1.1 and add the changelog entry

Covers everything merged since v2.1.0 so main is ready to tag. Nothing
under lib/ changed; the user-visible fix is the README, whose published
copy on pub.dev still shows a dead CI badge and a demo image linked from
a branch that no longer exists.

Co-Authored-By: Claude <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>