Refactor oneliner storage to use enum

The oneliner is now stored as `OnelinerState` enum which can represent
three states: `Ready`, `Empty` or `Error`. This allows the client to
differentiate between a case with no medical content and a case where
the oneliner generation failed.

The `OnelinerState` enum is serialized to JSON and stored in
`oneliner.json` file. The `OnelinerEntry` struct has been updated to
reflect this change.

The client-desktop application has been updated to handle the new
`OnelinerState` enum and display appropriate UI elements for each state.

The server-side code has also been updated to read and write the
`OnelinerState` enum, and to handle the new file format.
The `reset_case_artefacts` function in
`server/src/routes/case_actions.rs` has been updated to remove
`oneliner.txt` and create `oneliner.json` instead.

The tests have been updated to reflect these changes.

client-desktop/src/app.rs

@@ -18,6 +18,7 @@ use doctate_client_core::{
     PendingUpload, ServerSync, SnapshotCache, SyncConfig, UploadEvent, WorkerSnapshot, WorkerState,
     pick_footer_status, run_startup_cleanup, write_sidecar,
 };
+use doctate_common::oneliners::OnelinerState;
 use doctate_common::timestamp::{
     extract_hhmm, now_rfc3339, parse_rfc3339, recorded_at_to_filename_stem,
 };
@@ -506,11 +507,12 @@ impl DoctateApp {
                 ui.add_space(4.0);
 
                 let time_str = extract_hhmm(&marker.last_activity_at);
-                let oneliner = marker
-                    .oneliner
-                    .as_deref()
-                    .map(str::to_owned)
-                    .unwrap_or_else(|| "⏳".to_owned());
+                let oneliner = match &marker.oneliner {
+                    Some(OnelinerState::Ready { text, .. }) => text.clone(),
+                    Some(OnelinerState::Empty { .. }) => "∅".to_owned(),
+                    Some(OnelinerState::Error { .. }) => "⚠".to_owned(),
+                    None => "⏳".to_owned(),
+                };
                 let has_pending = pending_ids.contains(&marker.case_id);
                 let line = if has_pending {
                     format!("⬆ {time_str} · {oneliner}")