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/page_setup_import.rs

@@ -11,6 +11,7 @@ use std::path::{Path, PathBuf};
 /// The result names the file the way the chooser shows it.
 fn read_page_setups_task(path: PathBuf) -> Task<Message> {
     super::file::background_task(
+        crate::t!("Page setup import"),
         move || {
             let file = path
                 .file_name()