fix: detect 1337x invalid username response

[kuishou68]

Summary

• add the invalid username HTML snippet reported in issue 2815 to the 1337x site definition
• include both the capital-U and lowercase-u Bad username variants to avoid a fragile case-sensitive miss
• keep the change limited to site data so behavior for other providers stays untouched

Self-review

• re-checked the issue report before changing anything to confirm this is a site-data mismatch, not a core matcher bug
• kept the fix in the site data file instead of changing matching logic, because the failure is specific to 1337x response content
• validated the edited JSON parses cleanly after the change

Closes #2815

[github-actions]

Automatic validation of changes

Target	F+ Check	F- Check
1337x	❌   Fail	❌   Fail

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Failures were detected on at least one updated target. Commits containing accuracy failures will often not be merged (unless a rationale is provided, such as false negatives due to regional differences).