fix(A.2 #19): only report audio_path when the WAV writer finalises cleanly

Review feedback: reported_audio_path was cached at writer-open time
and survived a mid-session writer drop. If an append error cleared
wav_writer (or hound's Drop ran instead of finalize), the Summary
still reported the path — risking StopLiveTranscriptionResponse
pointing at a file whose header did not reflect its data chunk.

Moves the decision to end-of-session. wav_writer.take() is inspected:
- writer present + finalize Ok → report the path,
- writer present + finalize Err → None, emit a Warning,
- writer absent (mid-session drop, or never opened) → None.

StopLiveTranscriptionResponse.audio_path now means "the recording is
known-good" rather than "a recording was attempted". Users can still
recover partial files via filesystem if needed; the warning toasts
already emitted by append_resampled_audio on write failure surface
that path implicitly.

src-tauri/src/commands/live.rs

@@ -402,15 +402,12 @@ fn run_live_session(
         },
         None => None,
     };
-    // Resolve the reported audio_path only if the writer actually
-    // opened; otherwise the returned Summary must not claim a path.
-    let reported_audio_path: Option<String> = if wav_writer.is_some() {
-        audio_path
-            .as_ref()
-            .map(|p| p.to_string_lossy().to_string())
-    } else {
-        None
-    };
+    // `reported_audio_path` is decided at end-of-session based on
+    // whether the writer finalised successfully, not at open time.
+    // This way a writer that dies mid-session (append error clearing
+    // wav_writer, or finalise returning Err) does not leak a stale
+    // path back to the frontend that might point to a file whose
+    // header is out of sync with its data chunk.
 
     let mut buffer_start_sample: u64 = 0;
     let mut dropped_audio_ms: u64 = 0;
@@ -552,23 +549,32 @@ fn run_live_session(
         thread::sleep(Duration::from_millis(10));
     }
 
-    // Finalise the progressive WAV writer so the terminal header
-    // reflects every written sample. A failure here leaves the last
-    // flushed header state on disk — still a playable WAV — so we
-    // report a warning but don't fail the whole session.
-    if let Some(writer) = wav_writer.take() {
-        if let Err(e) = writer.finalize() {
-            let _ = status_channel.send(LiveStatusMessage::Warning {
-                session_id,
-                message: format!("WAV finalise failed: {e}; partial recording is still playable."),
-            });
-        }
-    }
+    // Finalise the progressive WAV writer and decide whether to
+    // report a path to the frontend. Only a clean finalise produces a
+    // reported path: a writer that died mid-session (wav_writer was
+    // already None) or a finalise that itself errored both yield
+    // `None`, so `StopLiveTranscriptionResponse.audio_path` reflects
+    // "recording is known-good" rather than "recording was attempted".
+    let audio_path = match wav_writer.take() {
+        Some(writer) => match writer.finalize() {
+            Ok(()) => audio_path.as_ref().map(|p| p.to_string_lossy().to_string()),
+            Err(e) => {
+                let _ = status_channel.send(LiveStatusMessage::Warning {
+                    session_id,
+                    message: format!(
+                        "WAV finalise failed: {e}. A partial recording may still be on disk from earlier flushes."
+                    ),
+                });
+                None
+            }
+        },
+        None => None,
+    };
 
     Ok(LiveSessionSummary {
         session_id,
         dropped_audio_ms,
-        audio_path: reported_audio_path,
+        audio_path,
     })
 }