feat(auth): #63: add session inactivity timeout and auto-lock

[imdeepmind]

Summary

Adds a configurable session inactivity timeout and auto-lock mechanism for the desktop application.

Changes

• Go Backend (internal/features/auth):
  • Track lastActiveAt on Session.
  • Add TouchSession() Wails binding to update lastActiveAt when user activity occurs.
  • Implement inactivity expiration check in GetSession(), RequireSession(), and CurrentClient(). Upon expiration, the session is cleared and the database connection is closed.
• Settings (internal/features/settings):
  • Add InactivityTimeoutMinutes (default: 15; options: 1 min, 5 min, 15 min, 30 min, 0/Disabled) to Settings struct and UpdateSettingsInput.
  • Sync timeout preference with auth.Service.
• React Frontend:
  • Add throttled activity listeners in AuthContext to ping TouchSession() every 10s when active.
  • Periodic background check every 5s to navigate user to Login screen when session expires.
  • Added Session Security & Auto-Lock UI in ApplicationSettings.tsx.

Closes #63