Sync and discard unauthenticated changes simple

[magush27]

Warn unauthenticated users on unlock and discard local changes on login
Simplified version of #2193.

Changes:

• Show a warning modal when a logged-out user unlocks the board to edit, informing them that changes are saved locally only and offering options to log in/sign up or continue editing. The modal is triggered only on the lock click, not on every individual edit action.
• Created a new UnauthenticatedEditModal component (standalone, no Redux) instead of modifying the existing LoginRequiredModal.
• On login, discard local unauthenticated changes (boards and communicators) if the user is not a first-time user or already has saved communicators on the backend, restoring their remote state instead.
• Fixed a stale activeBoardId bug (from Manage how to sync unauthenticated changes #2193 review): after discarding local boards, if the previously active board no longer exists in the new state, fall back to 'root' to avoid a blank board view.
• Boards from login response are now merged directly in the LOGIN_SUCCESS reducer case instead of via a separate addBoards dispatch.