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/derived_sync.rs

@@ -32,6 +32,7 @@
 //! share boundary the account cannot write to. The scanner excludes it by the
 //! same mechanism that excludes the trash.
 
+use crate::executors::{self, Executor};
 use std::path::{Path, PathBuf};
 
 use dr_sync::{Connection, RemoteBackend, RemoteError, RemoteId, RemotePath};
@@ -132,7 +133,7 @@ pub fn spawn_sync(
 ) -> std::sync::mpsc::Receiver<SyncMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "sync", move || {
         // A current-thread runtime here is what made the library scan itself
         // rather than adopt the shards the server already had; see net_runtime.
         let rt = match crate::net_runtime::build() {
@@ -1018,7 +1019,7 @@ pub fn spawn_place_fetch(
 ) -> std::sync::mpsc::Receiver<Result<Option<dr_types::Place>, String>> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "places", move || {
         let rt = match crate::net_runtime::build() {
             Ok(rt) => rt,
             Err(e) => {