Normalize PEP 621 project names

[fukalous]

Briefcase currently requires tool.briefcase.project_name even when the project already defines a PEP 621 project.name, and the new/convert wizards default to the formal name rather than a package-compatible name.

This change:

• normalizes project.name into Briefcase project_name metadata;
• preserves explicit legacy tool.briefcase.project_name values while warning when they are not PEP 621 compliant; and
• uses the validated app name as the default project name in the new and convert wizards.

Tests and configuration documentation have been updated to cover normalized, invalid, legacy, new, and convert paths.

Fixes #3011

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by: ChatGPT