fix(fuse,cli): bound the stop's wait for busy threads (B165)

A stop joined every background thread for as long as each took. The sync
engine's mirror pass calls Drive with no deadline, and a stop that waited for
it was killed by systemd at TimeoutStopSec. The stop now waits up to 10 s for
the background threads and FUSE workers together, names any still running, and
the runtime shuts down within 2 s.

crates/pdfs-cli/src/main.rs

@@ -2090,6 +2090,9 @@ fn mount_once(mountpoint: Option<PathBuf>) -> Result<pdfs_fuse::MountOutcome> {
     if let Err(e) = rt.block_on(auth::persist(&session)) {
         tracing::debug!(error = %e, "no new tokens to persist on shutdown");
     }
+    // Dropping the runtime waits for every blocking task, however long; a call
+    // still stuck on Drive would hold the stop until systemd kills it (B165).
+    rt.shutdown_timeout(std::time::Duration::from_secs(2));
 
     let outcome = result.context("mount failed")?;
     Ok(outcome)