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.

server/tests/analyze_test.rs

@@ -837,7 +837,11 @@ async fn reset_case_clears_derived_and_unfails_audio() {
     // A failed recording: raw audio written, but filename carries `.failed`.
     std::fs::write(dir.join("2026-04-15T09-05-00Z.m4a.failed"), b"audio-bytes").unwrap();
     // Derived artefacts the reset must wipe.
-    std::fs::write(dir.join("oneliner.txt"), "Knie re.").unwrap();
+    std::fs::write(
+        dir.join("oneliner.json"),
+        br#"{"kind":"ready","text":"Knie re.","generated_at":"2026-04-18T10:00:00Z"}"#,
+    )
+    .unwrap();
     std::fs::write(dir.join("document.md"), "# Doc\n").unwrap();
     std::fs::write(dir.join("analysis_input.json"), "{}").unwrap();
 
@@ -854,7 +858,7 @@ async fn reset_case_clears_derived_and_unfails_audio() {
     assert!(!dir.join("2026-04-15T09-05-00Z.m4a.failed").exists());
     // Derived artefacts gone.
     assert!(!dir.join("2026-04-15T09-00-00Z.transcript.txt").exists());
-    assert!(!dir.join("oneliner.txt").exists());
+    assert!(!dir.join("oneliner.json").exists());
     assert!(!dir.join("document.md").exists());
     assert!(!dir.join("analysis_input.json").exists());
 }