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/launch_ui.rs

@@ -4,6 +4,7 @@
 //! the develop window's wiring. The model holds the state machine and is
 //! tested headless; this module only moves values across the boundary.
 
+use crate::executors::{self, Executor};
 use std::cell::RefCell;
 use std::rc::Rc;
 
@@ -464,7 +465,7 @@ fn spawn_login(weak: slint::Weak<AppWindow>, ctl: Rc<LaunchController>, server:
     // the thread boundary.
     let (tx, rx) = std::sync::mpsc::channel::<LoginMessage>();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "login", move || {
         // A panic anywhere below would unwind the thread, drop `tx`, and leave
         // the UI with nothing but a closed channel — which it can only report
         // as "failed unexpectedly", losing the one piece of information that
@@ -533,7 +534,7 @@ fn spawn_direct_login(
 ) {
     let (tx, rx) = std::sync::mpsc::channel::<LoginMessage>();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "login", move || {
         let panic_tx = tx.clone();
         let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(move || {
             // Multi-thread for the reason the browser flow is: a current-thread
@@ -767,7 +768,7 @@ fn spawn_folder_list(weak: slint::Weak<AppWindow>, ctl: Rc<LaunchController>, pa
 
     let (tx, rx) = std::sync::mpsc::channel::<Result<Vec<String>, String>>();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "folders", move || {
         // Multi-thread for the same reason as the login worker: a
         // current-thread runtime left reqwest's connection future unpolled on
         // Android, so the await never resolved and the thread stopped without