[Feature] Add configurable default performer gender

[Stash-KennyG]

Fixes #3751

Summary

• add a new interface setting defaultPerformerGender (None + all gender enum values)
• apply the setting as a fallback when creating performers without explicit gender
• preselect default gender in new performer form, and apply it to non-stash scraper results when scraper gender is missing
• keep explicit user/scraper gender precedence and avoid mutating existing performers

Backend

• extend config GraphQL schema for interface input/result
• add config key/getter and wire configure/query plumbing
• apply fallback in performer create path
• add tests for explicit-vs-default precedence helper behavior

UI

• add setting in Interface > Performer List
• reuse shared gender list source
• fix locale key usage so dropdown shows localized labels (not raw keys)
• add English locale strings and changelog entry