Restore login account switching

[shiny-code-bot]

Finish Line

Users can switch the active login account without dropping queued work or stranding the TUI during app-server thread replacement.

Current Status

Deferred from lab.5 by explicit product decision. The release will not restore account switching merely to recover pre-alignment parity. Keep this issue parked for post-release dogfood; resume only if observed use justifies the session-replacement and rollback carrying cost. No implementation should land before lab.5 publication or the first post-release convergence checkpoint.

Acceptance Criteria

• Restore account-selection event handling.
• Guard switching while primary-thread events are pending.
• Preserve separate auth_home behavior and local/app-server backend distinctions.
• Reuse the reviewed replacement-thread helper from the removal stage.
• Restore the prior UI and account state on any mid-switch failure.
• Refresh the accounts view and active account display on success.
• Cover pending-work rejection, successful switch, and replacement failure.

Relationships

• Final implementation stage of Restore login account management TUI wiring #690.
• Blocked by Restore login account removal #697.

Open Questions

• Track profile-oriented /login arguments and /logout separately if they remain outside this stage.