feat(auth): support passkey (webauthn) login and credential management

[BiFangKNT]

Summary

Support WebAuthn Passkey passwordless login and credential management.

Key Changes

• Backend & Persistence:
  • Add webauthn dependency.
  • Add passkey_credentials table with Alembic migration 0024_passkey_credentials.
  • Implement single-use, TTL-pruned WebAuthn challenge lifecycle in UserService.
  • Implement registration options/verify, authentication options/verify, and passkey CRUD in UserService and UserRouterGroup.
  • Gracefully handle reverse-proxy origins and non-standard ports.
• Frontend & UI:
  • Add @simplewebauthn/browser dependency.
  • Add 1-click Passkey login button on the login page (web/src/app/login/page.tsx).
  • Add Passkey management section in the Account Settings dialog (AccountSettingsPanel.tsx) supporting registering devices, viewing keys, renaming, and revoking.
  • Complete i18n support (zh-Hans, en-US, ja-JP).
• Tests:
  • Add unit tests for challenge lifecycle and options generation.
  • Add API integration tests for public endpoints, protected endpoints, and reverse-proxy scenarios.

[codecov]

Codecov Report

❌ Patch coverage is 36.89320% with 195 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/langbot/pkg/api/http/controller/groups/user.py	15.59%	92 Missing ⚠️
src/langbot/pkg/api/http/service/user.py	48.75%	82 Missing ⚠️
...tence/alembic/versions/0024_passkey_credentials.py	0.00%	21 Missing ⚠️

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