refactor: centralize durable writes and split large modules

[devmobasa]

Summary

• Add shared durable_io atomic write helpers with explicit overwrite, permission, symlink, and sync policies.
• Route config, capture, onboarding, session backup, daemon/control, tray, Hyprland, and service/drop-in writes through durable write paths.
• Split large modules and colocate tests for daemon control/core, toolbar item config, capture/onboarding state, input helpers, render profiles, blur, and unix signal handling.
• Add tools/code-health-report.sh to track large files/functions and direct production
  fs::write usage.

Why

This makes persistence behavior consistent across the app, reduces scattered ad hoc write logic, and makes oversized modules easier to review and maintain without changing their public behavior.