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/duplicates.rs

@@ -38,6 +38,7 @@
 //! is a function of the image id, so the next review finds each such file
 //! there, and consolidating again treats it as already moved.
 
+use crate::executors::{self, Executor};
 use std::path::PathBuf;
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::mpsc::{Receiver, Sender};
@@ -492,7 +493,7 @@ pub fn spawn_check(
     stop: Arc<AtomicBool>,
 ) -> Receiver<DupMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "dupes", move || {
         let stopped = run_check(&conn, &catalog_path, cache_dir, groups, &stop, &tx).err();
         let _ = tx.send(DupMessage::Finished { stopped });
     });
@@ -758,7 +759,7 @@ pub fn spawn_consolidate(
     stop: Arc<AtomicBool>,
 ) -> Receiver<DupMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "consolidate", move || {
         let stopped = run_consolidate(&conn, &catalog_path, cache_dir, plans, &stop, &tx).err();
         let _ = tx.send(DupMessage::Finished { stopped });
     });