Start every worker thread through the executors module

Thirty-nine spawn sites in dr-ui, and one in the Android entry point,
called std::thread::spawn or a Builder of their own, and most of the
threads they started were <unnamed> in a panic message or a profiler.
Each now calls executors::spawn with its executor and a role, so the thread is
named <executor>:<role> — net:sync, decode:thumbs, io:catalog-open —
and knows which executor it is on. The three that already set a name
(automation, import, prefetch) keep their name as the role.

Behaviour is unchanged: each job still gets a thread of its own when it
starts, and spawn panics where std::thread::spawn did.

The module's documentation now says how a job is assigned: by what it
spends its time on, so a sweep that fetches bytes and then decodes them
is Decode, and a sidecar write that touches the catalog is Network.

Left as they were: the segmentation and refine workers in masks_ui.rs,
which another change is reworking, and test-only threads.

ui/dr-ui/src/library/scan.rs

@@ -1,6 +1,7 @@
 //! Walking a remote library into the catalog, and pulling other devices'
 //! judgements out of the sidecars the walk finds along the way.
 
+use crate::executors::{self, Executor};
 use dr_catalog::Catalog;
 use dr_sync::{Connection, RemoteBackend, RemoteId, RemotePath};
 use dr_types::FormatFilter;
@@ -77,7 +78,7 @@ pub fn spawn_scan(
 ) -> Receiver<ScanMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "scan", move || {
         let started = std::time::Instant::now();
         if let Err(e) = run_scan(&tx, conn, root, filter, catalog_path, started) {
             let _ = tx.send(ScanMessage::Failed {