Add 'Allow Multiple Logins' soft-switch (WIP)

[No0ne558]

This PR introduces an experimental soft-switch to permit the same employee to sign into multiple terminals concurrently.

• Adds Settings::allow_multi_login (persisted; SETTINGS_VERSION bumped to 107).
• Adds a UI switch "Allow Multiple Logins" in Settings (toggles and persists immediately).
• Terminal::LoginUser() now respects settings->allow_multi_login before rejecting a login for an employee already online.
• Login UI (LoginZone) checks updated to avoid blocking when the switch is enabled.
• Changelog updated with WIP note.

This is a work-in-progress feature and should be tested on staging. Shared per-employee mutable state (e.g., current_job, last_job) remains global — enabling multi-login will not isolate per-terminal session state.

• Test (default OFF):
  • Start server and open two terminal clients.
  • Log in user A on Terminal 1.
  • Attempt to log in user A on Terminal 2 — Terminal 2 should be blocked with "You're Using Another Terminal".
• Test (enable ON):
  • In Settings -> Switches, toggle Allow Multiple Logins to On (persisted immediately).
  • Repeat login: log user A on Terminal 1, then log in user A on Terminal 2 — both should succeed.
• Verify:
  • Clocking, open checks, and logout behaviors on both terminals.
  • Confirm settings.dat contains the new allow_multi_login value (after toggle/save).
  • Try deleting a user while logged in — deletion should remain blocked.

Notes / Risks

• WIP: This feature is experimental — please test thoroughly.
• Shared state: Employee fields remain shared across terminals; enabling multi-login may lead to surprising behavior for per-user mutable state.
• Settings migration: SETTINGS_VERSION bumped to 107 — verify settings load/save across upgrades and backups.
• Concurrency: Race conditions around IsUserOnline are possible under heavy concurrent logins; consider additional synchronization if needed.
• Revert: To revert the change locally: git revert cf62abe (or checkout individual files from previous commit).