feat(daemon): Linux D-Bus integration — notifications, Secret Service, systemd

[igouss]

Summary

Implements Linux-native integration for the GSD daemon (M005):

• DBusEventBridge — desktop notifications via org.freedesktop.Notifications on session lifecycle events
• DBusBridgeBlocker — action-button blocker toasts via org.freedesktop.portal.Notification (requires systemd service registration)
• SecretServiceAdapter — credential storage/retrieval via org.freedesktop.secrets (GNOME Keyring / KDE Wallet), with 2s timeout and null-on-any-failure contract
• GApplicationService — GApplication lifecycle management for portal access
• linux.ts — systemd user service unit generation and installation helpers
• cli.ts — --keyring flag defaults to true on Linux; new subcommands: install-service, secret-set, secret-get

Design constraints upheld

• D-Bus unavailability never crashes or delays the daemon (R005)
• getCredential returns null on any failure — never throws
• bus.disconnect() always runs in finally to avoid ERR_STREAM_WRITE_AFTER_END

Branch

feat/m005-daemon-linux-dbus-secretservice