fix(fuse,cli): recover the mount after a watchdog kill

A daemon killed by the watchdog left its FUSE mounts dead. A dead mount
answers ENOTCONN, which create_dir_all took for an absent directory, so
every restart failed with EEXIST and retried forever without reaching the
code that clears stale mounts. mount_once now goes through
prepare_mountpoint, which clears a dead mount first.

Startup on a large account (70 s hydrating 48k nodes) plus the supervisor's
first ping one full period later outlasted WatchdogSec=120, so a restart
that did mount was aborted again. The daemon now asks for a 10-minute
watchdog before startup, the supervisor pings on its first tick, and the
unit's own timeout is restored once the control socket answers.

Records B279.

crates/pdfs-cli/src/main.rs

@@ -2520,7 +2520,10 @@ fn mount_once(mountpoint: Option<PathBuf>) -> Result<pdfs_fuse::MountOutcome> {
     // An explicit `--mountpoint` wins; otherwise honor the Settings-page choice
     // (config), falling back to the default location.
     let mountpoint = mountpoint.unwrap_or_else(|| dirs.resolved_mountpoint(&config));
-    std::fs::create_dir_all(&mountpoint)
+    // Hydrating a large account and remounting its folders can outlast the
+    // unit's watchdog before the supervisor sends its first ping (B279).
+    pdfs_fuse::stretch_watchdog_for_startup();
+    pdfs_fuse::prepare_mountpoint(&mountpoint)
         .with_context(|| format!("create mountpoint {}", mountpoint.display()))?;
 
     let username = auth::load().map(|s| s.username).unwrap_or_default();