2.0.9: Standardize admin-only language keys with admin. prefix

- admin.core.* -> admin.* (position admin.core -> admin.common)
- admin_notice.* -> admin.notice.* (static labels + dynamic notice content)
- display.* -> admin.display.*
- password_policy.admin.* -> admin.password_policy.* (incl. config detail column)

Route names (admin_notice.*) are untouched. Modification 20260716T140000,
NFR-MAINT-admin-language-key-prefix.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>