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/automation.rs

@@ -45,6 +45,7 @@
 //!
 //! Waiting is the client's job (poll `locate`), which keeps this stateless.
 
+use crate::executors::{self, Executor};
 use std::io::{BufRead, BufReader, Write};
 use std::os::unix::net::{UnixListener, UnixStream};
 use std::sync::mpsc;
@@ -71,15 +72,13 @@ pub(crate) fn attach(window: &AppWindow) {
     };
     log::info!("automation: listening on {path:?}");
     let weak = window.as_weak();
-    std::thread::Builder::new()
-        .name("automation".into())
-        .spawn(move || {
-            for stream in listener.incoming().flatten() {
-                let weak = weak.clone();
-                std::thread::spawn(move || serve(stream, weak));
-            }
-        })
-        .ok();
+    executors::try_spawn(Executor::Io, "automation", move || {
+        for stream in listener.incoming().flatten() {
+            let weak = weak.clone();
+            executors::spawn(Executor::Io, "auto-conn", move || serve(stream, weak));
+        }
+    })
+    .ok();
 }
 
 fn serve(stream: UnixStream, weak: slint::Weak<AppWindow>) {