Move username sanitization to model and make it less strict

[Boy132]

Closes #2138

Before:
Backend silently removed . and - from username, made it lowercase and max length was 64.
Frontend did not check these restrictions.

Now:
Username is only made lowercase. (to allow case insensitive login) . and - are allowed. (will cause no problems with sftp)
Frontend and backend limits for max length match.

[coderabbitai]

📝 Walkthrough

Walkthrough

The changes modify username normalization behavior by removing the character sanitization chain that stripped special characters during user creation, while simplifying lowercase enforcement to occur uniformly at the model persistence layer for both username and email fields.

Changes

Cohort / File(s)	Summary
Model Persistence Layer app/Models/User.php	Modified the booted saving behavior to lowercase both username and email using Str pipeline during save, consolidating case normalization at persistence time instead of relying solely on the existing email-only lowercasing via mb_strtolower.
User Creation Service app/Services/Users/UserCreationService.php	Removed the username normalization chain that previously sanitized usernames by replacing periods and hyphens, converting to ASCII, and truncating to 64 characters. The generated default username now bypasses these transformations.

Possibly related PRs

• #1731: Modifies username normalization in app/Models/User.php by enforcing lowercase/trim on username fields
• #1702: Removes the Username validation rule and related UI, complementing the shift away from character sanitization enforcement

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: sanitization logic is relocated to the model and constraints are relaxed.
Description check	✅ Passed	The description clearly explains the before/after behavior and addresses the issue: less strict sanitization and frontend-backend alignment.
Linked Issues check	✅ Passed	The code changes meet issue #2138 requirements by allowing dashes and preserving case via lowercase-only sanitization, directly addressing the reported problems.
Out of Scope Changes check	✅ Passed	Both file changes directly address the linked issue: removing strict sanitization from the service and moving lowercase-only normalization to the model layer.

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