feat(gui): add pdfs-prompt --preload for session autostart

--preload registers the resident GTK launcher and realizes its window
without showing it, so the first shortcut opens it as fast as later
ones. When a launcher is already running it exits without activating
it.

The application hold now happens in startup, which only the primary
instance runs. Taken before registration, it also kept every remote
invocation alive after it had forwarded its activation.

docs/USER_GUIDE.md

@@ -235,6 +235,15 @@ The window stays loaded after the first use, so later shortcuts open it instantl
 and video open straight from the Proton Drive folder, so players can start before the whole file
 downloads. Other Drive files download first, then open.
 
+To make the first shortcut instant as well, start the launcher hidden with your session.
+`pdfs-prompt --preload` loads the window without showing it and does nothing if the launcher is
+already running. For Hyprland:
+
+```ini
+exec-once = pdfs-prompt --preload
+bind = SUPER, space, exec, pdfs-prompt
+```
+
 ### Using your own launcher
 
 `pdfs-prompt --dmenu` shows the results in fuzzel, rofi, wofi, tofi, bemenu or dmenu, whichever is