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/lib.rs

@@ -747,7 +747,7 @@ fn drain_outbox(library: &Rc<library_ui::LibraryController>) {
 
     let root = conn.account.root.clone();
     let rx = export::spawn_upload(conn, root, outbox);
-    std::thread::spawn(move || {
+    executors::spawn(executors::Executor::Io, "upload-log", move || {
         while let Ok(msg) = rx.recv() {
             match msg {
                 export::UploadMessage::Status(s) => log::info!("export: {s}"),