feat(app): windows run-elevated-on-login

elevation.rs implements the Windows one-click "run elevated on login" + "restart
elevated now" (ROADMAP M3.5 deferred to M5). set_run_elevated_on_login registers
/ removes a Task Scheduler task via schtasks (/Create /TN ... /TR "<exe>
--minimized" /SC ONLOGON /RL HIGHEST /F; /Delete treats not-found as idempotent),
is_run_elevated_on_login_enabled queries it, restart_elevated relaunches elevated
via PowerShell Start-Process -Verb RunAs then exits (fail-safe: stays running if
UAC is declined). cfg(windows)-gated with compiling non-Windows arms; failures map
to a single-line, secret-free anyhow error the IPC CommandError serializes.
Reachable-but-uncalled until the M6 settings IPC (allow(dead_code) at the module
declaration). Live round-trip test honestly gate-skips when not elevated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CyiRqk2DVwmJjEu5gcD1m

Author: pmaxhogan