fix(cli,core,fuse,gui): say why the mount keeps failing, and back off

A mount that failed on every try retried every 5 s forever, and the
reason was only in the journal: the app and the tray said Connecting...
for days (B263, issue #40). The retry now doubles to a 2-minute ceiling,
and the last failure is kept in mount-failure.json for the app, the
tray, pdfs diagnose and systemctl status.

The early returns in mount left the workers they had started running
beside the next attempt (B264). They are now stopped on every error
path, without telling systemd the daemon is stopping.

crates/pdfs-core/src/lib.rs

@@ -15,6 +15,7 @@ pub mod kernelkey;
 pub mod keycheck;
 pub mod localindex;
 pub mod menu;
+pub mod mountfail;
 pub mod mounts;
 pub mod opener;
 pub mod places;