Fix sidecar pipe crash on state changes and show logged-in state in settings

The state callback in main_headless.py wrote events to stdout
synchronously, so an EINVAL on the Tauri sidecar pipe (Windows) bubbled
up through _set_state and tore down engine init and reload_engine. That
turned PUT /api/config into a "Failed to fetch" for the user. The print
is now pipe-safe and api_server isolates the chained callback so a
future misbehaving listener cannot break the engine state machine.

Settings also now persists remote.email on login and shows a "Logged in
as <email>" indicator with a Log out button when an auth_token is
present, instead of leaving the email/password fields blank on reload.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/default_config.yaml

@@ -72,6 +72,7 @@ remote:
   mode: byok  # local | managed | byok
   server_url: "https://transcribe.shadowdao.com"  # Proxy server URL for managed mode
   auth_token: ""  # JWT stored after login (managed mode)
+  email: ""  # Email of the logged-in managed-mode account (for UI display)
   byok_api_key: ""  # Deepgram API key for BYOK mode
   deepgram_model: nova-2  # Deepgram model to use
   language: en-US  # Language code