feat: add password reset shortcut and auth doc cleanup

[jasper0507]

Summary

• Add a settings-menu shortcut that opens the forgot-password flow with the logged-in account email prefilled.
• Carry auth reset state through the app so the auth screen can reopen in password-reset mode.
• Clean up the real-data-loop design doc to use current diaryKey terminology and note the modern session model.
• Add regression coverage for the password-reset shortcut.

Test Plan

• cd frontend && npm test
• cd frontend && npm run build

Notes

• Includes an Unreleased changelog entry for the new auth UX polish.