Browser extension: detect and subscribe to standard.site publications on ATProto

feat: revoke a switched-away session only after the new sign-in lands master

The worker now checks the stored mirror when a token exchange completes: if the new session belongs to a different account, the old session is revoked through the offscreen OAuth host and its cached subscriptions are dropped. Re-signing into the same account revokes nothing, since the fresh session lives under the same sub. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>