Fix partial username autofill and legacy alias detection

[Iamsujithd]

What changed

• accept a case-insensitive typed username prefix during the protected credential handoff
• complete the selected saved username before filling its password
• preserve exact case-insensitive matches and reject genuinely divergent usernames
• route strongly identified registration email fields directly to private-email handling
• support legacy registration forms whose email controls lack name, id, placeholder, and autocomplete metadata
• retain exclusions for login, recovery, newsletter, contact, and ambiguous email fields

Verification

• exact public Register.html structure inspected and covered by regression test
• focused autofill suite: 49 tests passed
• full repository pipeline: 38 files / 262 tests passed
• lint and typecheck passed
• Chrome and Firefox production builds and release ZIPs passed
• artifact, secret, size, source-map, permission, and SBOM verification passed