Prepare Flet 0.83.0 release

[FeodorFitsner]

Summary

Prepare the 0.83.0 release.

• bump packages/flet version to 0.83.0
• add 0.83.0 entries to root and Flutter package changelogs
• refresh the client lockfile to reference flet 0.83.0
• update the release-prep skill to require fixing missing release milestones on GitHub instead of dropping issue links

Notes

• related issues #5912 and #6304 were updated to milestone 0.83.0
• commit also fixes an old changelog typo that blocked the typos pre-commit hook

Summary by Sourcery

Prepare the Flet 0.83.0 release by updating package metadata, changelogs, and release workflow guidance.

New Features:

• Document new 0.83.0 features including customizable scrollbars, enhanced ExpansionPanelList behavior, and expanded SharedPreferences value types.

Bug Fixes:

• Document 0.83.0 bug fixes covering cross-language default alignment, redundant page updates, ReorderableListView callbacks, and Pyodide dependency loading behavior.
• Correct an older tooltip attribute description typo in the changelogs.

Enhancements:

• Clarify the release preparation skill to require fixing missing GitHub milestones instead of dropping related issue or PR links from changelogs.

Build:

• Bump the Flutter package version to 0.83.0 and refresh the client lockfile to reference the new release.

Documentation:

• Add 0.83.0 entries to the root and Flutter package changelogs describing new features, improvements, and fixes.

[sourcery-ai]

We've reviewed this pull request using the Sourcery rules engine

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[ndonkoHenri]

Class validation chore missing in changelog.