LR-BEHAV-BG-TASKS: background export with progress and cancel

- Engine: export split into prepare_export (session thread, cheap) and PreparedExport::run
  (Send: read/decode/render/encode); run_batch with per-photo progress, cancel and collected
  errors; export_batch keeps the stop-at-first-error behaviour for scripted calls.
- Desktop: the Export dialog, Export with Preset (menu and context menu) and Export with
  Previous run their batch on a worker thread (app.export {background: true}); a progress
  panel shows count / file / Cancel; ui.inspect reports export {running, last}.
- Headless: settle waits for a running export; `lightcraft-cli snapshot` finishes it before
  exiting. Web keeps foreground exports (downloads are main-thread).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/lightcraft-cli/src/main.rs

@@ -587,6 +587,12 @@ fn snapshot(args: &[String]) -> Result<(), String> {
         eprintln!("lightcraft-cli snapshot: imported {n} files in {:.0} ms", ti.elapsed().as_secs_f64() * 1e3);
     }
     let services = lightcraft_ui_egui::Services {
+        write_shared: Some(std::sync::Arc::new(|p: &str, b: &[u8]| {
+            if let Some(dir) = Path::new(p).parent().filter(|d| !d.as_os_str().is_empty()) {
+                std::fs::create_dir_all(dir).map_err(|e| e.to_string())?;
+            }
+            std::fs::write(p, b).map_err(|e| format!("{p}: {e}"))
+        })),
         write: Some(Box::new(|p: &str, b: &[u8]| {
             if let Some(dir) = Path::new(p).parent().filter(|d| !d.as_os_str().is_empty()) {
                 std::fs::create_dir_all(dir).map_err(|e| e.to_string())?;
@@ -672,6 +678,15 @@ fn snapshot(args: &[String]) -> Result<(), String> {
         }
         eprintln!("lightcraft-cli snapshot: wrote {path} ({}×{})", r["result"]["width"], r["result"]["height"]);
     }
+    // a background export started by the script finishes before we exit (its files would be cut off)
+    if h.app.export.is_some() {
+        eprintln!("lightcraft-cli snapshot: waiting for the background export");
+        let te = std::time::Instant::now();
+        while h.app.export.is_some() && te.elapsed() < std::time::Duration::from_secs(3600) {
+            h.step();
+            std::thread::sleep(std::time::Duration::from_millis(20));
+        }
+    }
     eprintln!("lightcraft-cli snapshot: done in {:.2} s ({} frames)", t0.elapsed().as_secs_f64(), h.frames());
     Ok(())
 }