Prevent system theme sync from crashing after Pi reloads

[GodGardensGuns]

Issue

Pi now marks extension contexts as stale after session replacement or reload. pi-system-theme keeps a polling interval that captures the ctx from session_start. If Pi reloads, forks, switches sessions, or otherwise shuts down the current extension runtime while an appearance detection poll is in flight, that async poll can resume later and access ctx.ui.

That raises:

Error: This extension ctx is stale after session replacement or reload.

In interactive mode this can crash Pi via an uncaught exception.

Fix

• track the active session with a per-session token
• clear polling and invalidate the token on session_shutdown
• re-check the token after async appearance detection before reading or writing ctx.ui
• keep /system-theme save/apply behavior guarded against stale sessions as well

Testing

• npx --yes vitest run

Also added a regression test that simulates shutdown while appearance detection is in flight and verifies the stale ctx.ui is not accessed.