Fix app name validation logic and improve error message #5965

Job	Run time
Pre-commit checks / Pre-commit Checks	27s
Package Automation / Create Python Package	20s
Check towncrier / Towncrier Checks	28s
Package Briefcase / Create Python Package	15s
Unit tests (macos-latest, 3.9)	1m 29s
Unit tests (macos-13, 3.13)	2m 39s
Unit tests (ubuntu-24.04, 3.13)	1m 51s
Unit tests (windows-latest, 3.9)	3m 21s
Unit tests (ubuntu-24.04, 3.11)	1m 36s
Unit tests (macos-13, 3.9)	2m 21s
Unit tests (macos-latest, 3.13)	1m 16s
Unit tests (ubuntu-24.04, 3.9)	1m 36s
Unit tests (ubuntu-24.04, 3.10)	1m 36s
Unit tests (ubuntu-24.04, 3.12)	1m 50s
Unit tests (windows-latest, 3.13)	3m 7s
Project coverage	43s
Verify project	0s
Build app	0s
	24m 55s

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