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/thumbnails_gen.rs

@@ -1,6 +1,7 @@
 //! Generating thumbnails locally from a decoded preview or original, and
 //! the metadata that comes along for the ride.
 
+use crate::executors::{self, Executor};
 #[cfg(test)]
 use dr_catalog::Catalog;
 use dr_sync::{Connection, RemoteBackend, RemoteId, RemotePath};
@@ -101,7 +102,7 @@ pub fn spawn_thumbnails(
     // The one place this job names a decoder; everything below takes it.
     let decoder = dr_decode::default();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Decode, "thumbs", move || {
         let mut store = match ThumbStore::open(&store_dir) {
             Ok(s) => Some(s),
             Err(e) => {