fix(background): keep the app alive when a background worker panics

Bug:
background_task joined the worker thread with .expect(...). Any panic
inside a background closure (decoding a malformed file, an unexpected
None) was re-raised on the main thread and took the whole application
down, even though the worker's failure was already contained.

Reproduce:
1. Run an operation that executes in a background worker, e.g. loading
   or converting a deliberately corrupted file whose decoder panics.
2. Before: the process dies with the worker's panic message.
3. After: the command line shows "background task failed: <label> -
   <panic>" and the application keeps running.

Fix:
The worker closure is wrapped in catch_unwind; a panic is converted to
Message::BackgroundTaskFailed(context, message) delivered back to the
update loop and surfaced as a command-line error. Each call site passes
a context label naming the operation. A failed thread join (thread
aborted) is handled the same way instead of expect().

src/app/update/mod.rs

@@ -5288,6 +5288,7 @@ impl OpenCADStudio {
                 let worker_display = display_name.clone();
 
                 file::background_task(
+                    crate::t!("WBLOCK save"),
                     move || {
                         let out_doc = match source_mode {
                             WblockSourceMode::Block => {
@@ -9717,6 +9718,12 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::BackgroundTaskFailed { context, detail } => {
+                self.command_line
+                    .push_error(crate::tf!("{context} failed: {detail}").as_ref());
+                Task::none()
+            }
+
             // ── Print to system printer ───────────────────────────────────────
             Message::PrintToPrinter => self.on_print_to_printer(),
             Message::PrintResult(Ok(printer)) => {