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_fetch.rs

@@ -2,6 +2,7 @@
 //! cache, dehydration, and the prefetcher that keeps the grid ahead of
 //! scrolling.
 
+use crate::executors::{self, Executor};
 use crate::sidecar_cache::SidecarCache;
 use dr_catalog::Catalog;
 #[cfg(test)]
@@ -136,7 +137,7 @@ pub fn spawn_pin_fetch(
 ) -> Receiver<PinMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "pin", move || {
         let (store, catalog) = match (
             dr_catalog::Cache::open(&cache_dir, budget),
             Catalog::open(&catalog_path),
@@ -324,7 +325,7 @@ pub fn spawn_dehydrate(
 ) -> Receiver<usize> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "dehydrate", move || {
         let Ok(catalog) = Catalog::open(&catalog_path) else {
             return;
         };
@@ -442,7 +443,7 @@ pub fn spawn_sidecar_fetch(
 ) -> Receiver<Option<dr_pipeline::Sidecar>> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "sidecars", move || {
         let cache = SidecarCache::open(cache_dir);
         let path_str = sidecar_path(&image_path);
 
@@ -565,7 +566,7 @@ pub fn spawn_full_fetch(
     cache: Option<CacheContext>,
 ) -> Receiver<Result<Vec<u8>, FetchFailure>> {
     let (tx, rx) = std::sync::mpsc::channel();
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "fetch", move || {
         let _ = tx.send(fetch_original(conn, &path, cache.as_ref()));
     });
     rx
@@ -845,10 +846,10 @@ impl Prefetcher {
         let shared = std::sync::Arc::new(PrefetchShared::default());
         let (tx, events) = std::sync::mpsc::channel();
         let worker = shared.clone();
-        std::thread::Builder::new()
-            .name("prefetch".into())
-            .spawn(move || serve_prefetches(&worker, &tx))
-            .expect("spawning the prefetch worker");
+        executors::try_spawn(Executor::Network, "prefetch", move || {
+            serve_prefetches(&worker, &tx)
+        })
+        .expect("spawning the prefetch worker");
         Self { shared, events }
     }