Fix silent watch recordings: async session activation, encoder settings, error surfacing

Watch recordings arrived on the phone as 28-byte header-only m4a files
(record() reported success but no audio was ever captured). Activate the
audio session via watchOS's async activate(options:), drop the .spokenAudio
mode experiment, match the phone recorder's proven AAC settings
(44.1 kHz / 48 kbps), and surface silent failures: an AVAudioRecorderDelegate
catches encode errors mid-take, and a stop-time guard detects a dead
header-only file, deletes it, and shows the failure on the watch instead of
shipping an unplayable note to the phone.

Claude-Session: https://claude.ai/code/session_014esDWi42URLEC6Cj17hGQ3

Notes Watch App/WatchAppServices.swift

@@ -21,6 +21,11 @@ final class WatchAppServices {
         recorder.onInvoluntaryFinish = { [weak self] recording in
             self?.transfer.enqueue(recording)
         }
+        // A capture that silently produced no audio (encode error, dead input)
+        // surfaces on screen — there's no console on a real watch.
+        recorder.onCaptureFailure = { [weak self] message in
+            self?.lastError = message
+        }
     }
 
     /// Single-button behaviour: start if idle, stop-and-queue if recording.
@@ -48,7 +53,7 @@ final class WatchAppServices {
             return
         }
         do {
-            try recorder.startRecording()
+            try await recorder.startRecording()
             lastError = nil
         } catch {
             logger.error("could not start recording: \(error, privacy: .public)")