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/trash.rs

@@ -27,6 +27,7 @@
 //! halfway with no record of where it stopped is worse than one that reports
 //! "38 of 40".
 
+use crate::executors::{self, Executor};
 use std::path::PathBuf;
 use std::sync::mpsc::Receiver;
 
@@ -169,7 +170,7 @@ pub fn spawn_move(
 ) -> Receiver<TrashMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "trash", move || {
         let total = moves.len();
         let rt = match crate::net_runtime::build() {
             Ok(rt) => rt,
@@ -281,7 +282,7 @@ pub fn spawn_purge(
 ) -> Receiver<TrashMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "purge", move || {
         let total = paths.len();
         let rt = match crate::net_runtime::build() {
             Ok(rt) => rt,