fix: use delay mode and release power inhibitor on shutdown

[nvandamme]

Problem

On Linux, while any mount/job is active, RClone Manager acquires a logind
inhibitor lock with MODE=block. This causes two issues:

1. systemd-logind refuses ManagerReboot/Shutdown entirely — on KDE Plasma,
   clicking "Restart" silently degrades into a black screen, waiting indefinitely instead of rebooting.
   The reboot transaction never starts.
2. The PrepareForShutdown=true signal handler only logged a warning and never
   released the lock, so the lock survived until process exit even during shutdown.

Fix

• Change the inhibitor mode from "block" to "delay": logind proceeds with
  the reboot after its grace period instead of refusing it outright. A user app
  cannot truly block power-off anyway (power button, Ctrl+Alt+Del, remote admin).
• On PrepareForShutdown=true, actually call state.release() so the lock is
  released as soon as the OS begins shutting down — mirroring what already happens
  when all mounts are unmounted.

Verification

Tested on CachyOS (systemd 261, Plasma 6): with kDrive mounted and the patched
binary running, ListInhibitors now shows mode "delay", and KDE Restart performs
a real reboot instead of logging out.

[Hakanbaban53]

Hi @nvandamme,

Thank you for opening this PR and highlighting the shutdown hang on KDE Plasma / systemd!

We looked deeply into this behavior across different desktop environments and inhibitor modes:

1. The limitation with mode = "delay" on sleep:
   Systemd's delay inhibitor mode enforces a strict global timeout (typically 5 seconds). If sleep is set to delay, systemd only delays suspension for 5 seconds before forcing the system to sleep anyway — which interrupts long-running file transfers and syncs.
2. The issue with mode = "block" on sleep:
   Setting sleep with block requires Polkit privileges (org.freedesktop.login1.inhibit-block-sleep), which triggers root password prompts when a user manually clicks "Sleep" or locks the screen, and prevents sleep when closing a laptop lid.

Solution Implemented

To solve both the shutdown hang and the sleep handling cleanly, we implemented a comprehensive fix:

1. Switched to what = "idle", mode = "block":
   • Universal and unprivileged (no Polkit password prompts).
   • Prevents the OS from automatically going to sleep while active syncs, mounts, or jobs are running.
   • Allows passwordless manual sleep, screen locking, and hardware laptop lid-close sleep without interference.
2. Integrated with PrepareForShutdown(true) on D-Bus:
   • When an OS shutdown or reboot is initiated, Rclone Manager receives the logind signal, triggers our graceful shutdown sequence (handle_shutdown to unmount FUSE remotes and terminate jobs), and immediately releases the inhibitor lock. This eliminates any black-screen hangs or delayed power-offs.

Because this required reworking the inhibitor target and adding the D-Bus signal listener, we have implemented this fix directly in the codebase (targeted for the upcoming release).

Closing this in favor of the integrated solution. Thank you again for your valuable contribution and for helping us diagnose this!

Fixes in this commit:
e510205