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/sidecar.rs

@@ -1,6 +1,7 @@
 //! Writing local edits out to the catalog's sidecar outbox, and draining
 //! that outbox to the remote once a connection is available.
 
+use crate::executors::{self, Executor};
 use crate::sidecar_cache::SidecarCache;
 use dr_sync::{Connection, RemoteBackend, RemoteError, RemoteId, RemotePath};
 use std::path::PathBuf;
@@ -138,7 +139,7 @@ pub fn spawn_sidecar_writes(
 ) -> Receiver<SidecarMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "sc-write", move || {
         let cache = SidecarCache::open(cache_dir);
 
         // The runtime and the backend are only needed to *upload*. Offline,
@@ -451,7 +452,7 @@ pub(super) async fn write_one_sidecar_online(
 pub fn spawn_outbox_drain(conn: Connection, cache_dir: PathBuf) -> Receiver<SidecarMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "outbox", move || {
         let cache = SidecarCache::open(cache_dir);
         let queued = cache.pending();
         if queued.is_empty() {