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_ui/sync.rs

@@ -7,6 +7,7 @@
 //! library rather than in response to what the grid is showing.
 //! See `docs/dev/code-health.md` CH-1.
 
+use crate::executors::{self, Executor};
 use std::rc::Rc;
 
 use slint::ComponentHandle;
@@ -33,7 +34,7 @@ fn spawn_scheduled_backup(ctl: &Rc<LibraryController>) {
     if !dr_catalog::recovery::backup_due(&catalog_path) {
         return;
     }
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Io, "backup", move || {
         let catalog = match dr_catalog::Catalog::open(&catalog_path) {
             Ok(c) => c,
             Err(e) => {
@@ -111,7 +112,7 @@ pub(super) fn start_derived_sync(window: &AppWindow, ctl: &Rc<LibraryController>
         let outbox = crate::export::outbox_dir(&conn.account);
         if crate::export::pending_count(&outbox) > 0 {
             let rx = crate::export::spawn_upload(conn.clone(), conn.account.root.clone(), outbox);
-            std::thread::spawn(move || {
+            executors::spawn(Executor::Io, "upload-log", move || {
                 while let Ok(msg) = rx.recv() {
                     match msg {
                         crate::export::UploadMessage::Status(s) => log::info!("export: {s}"),