Add visual examples to README and fix validation on landing website

[KamilSztandur]

The sign-up demo accepted ada @example.com

The isEmail every example carried was value.contains('@'), so the first form a visitor touches on the landing page let spaces through. Not a great first impression for a validation library.

Every copy now matches a pattern — no whitespace, one @, a dot in the domain — in the landing hero, the custom-validators page (built on its own matches helper, so the composition it teaches does the work), server errors, both subform examples, and the example app. The test snippet in the testing docs gained the whitespace case.

Verified in a browser against the live islands:

typed	result
ada @example.com	rejected
ada@example	rejected
ada	rejected
ada@example.com	accepted

README

• The opening was all architecture, no benefit. It now leads with simple and complicated forms wired the same way, validation and async checks in the box, and keeps the technical sentence with its search keywords right underneath.
• Three unlabelled code blocks in a row under "Your first form" — each now says what it is: the form controller, the form widget, the field widget.
• A wall of text with nothing to look at, and no sign of what the snippets produce. Three screenshots now sit between the prose, taken from the live examples that are compiled from those very snippets: the first form after an empty submit, a username rejected by a server-side check, and a guest list where each guest is a subform added at runtime. Each links to its live version on the site.

The logo geometry Piotr also raised is left alone — a designer is on it.

Release

0.2.1+1, documentation only, no library changes. Version bumped in pubspec.yaml, example/pubspec.yaml and the example lockfile, with a CHANGELOG.md entry.

Checks

flutter analyze and flutter test (39) in the package and the island bundle, dart format, examples:check, examples:format:check, types:check, lint, format, next build, and a pass over every page whose snippet changed to confirm its island still comes up clean. pana reports 160/160 at the new version.

🤖 Generated with Claude Code

[github-actions]

Docs preview: https://advanced-forms-njq6gz8u6-leancode.vercel.app

Built from 6c9bec1; the landing page is at /, the docs under /docs.

[mateusz-pietras]

lgmt, check that one png tho