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/remote_folders.rs

@@ -9,6 +9,7 @@
 //! locally: the server may have normalised it, or refused it, and the list
 //! the user picks from should be the server's, not this app's guess at it.
 
+use crate::executors::{self, Executor};
 use std::cell::RefCell;
 use std::rc::Rc;
 use std::sync::mpsc::TryRecvError;
@@ -105,7 +106,7 @@ where
 {
     let work: Work = Box::new(work);
     let (tx, rx) = std::sync::mpsc::channel::<Result<Vec<String>, String>>();
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "remote-dirs", move || {
         // Multi-thread, for the reason the login worker records: a
         // current-thread runtime left reqwest's connection future unpolled on
         // Android, and the await never resolved.