feat(mobile): a meeting recording survives calls, silenced capture, low space and a deleted note

src/App/Backlog.Mobile.UI/Components/NoteRecordingBar.razor

@@ -15,13 +15,24 @@
 
 @if (Recording.IsRecording(NoteId))
 {
-    <RecordingBar State="@BarState" Elapsed="@Recording.Elapsed" Level="@Recording.Level"
+    <RecordingBar State="@BarState" Elapsed="@Recording.Elapsed" Level="@Recording.Level" Notice="@Recording.Notice"
                   OnPause="PauseAsync" OnResume="ResumeAsync" OnStop="StopAsync" TestId="@TestId" />
 }
-else if (Recording.LastFailure is { } failure && failure.NoteId == NoteId)
+else
 {
-    <RecordingBar State="RecordingBarState.Failed" Elapsed="@failure.Elapsed" Error="@failure.Error"
-                  OnDismiss="() => Recording.DismissFailure(NoteId)" TestId="@TestId" />
+    @if (Recording.LastFailure is { } failure && failure.NoteId == NoteId)
+    {
+        <RecordingBar State="RecordingBarState.Failed" Elapsed="@failure.Elapsed" Error="@failure.Error"
+                      OnDismiss="() => Recording.DismissFailure(NoteId)" TestId="@TestId" />
+    }
+
+    @* The note was deleted elsewhere while it recorded; Stop made a new note for the
+       recording, and this page is the one place the person still looks for it. *@
+    @if (Recording.LastMoved is { } moved && moved.FromNoteId == NoteId)
+    {
+        <Alert Message="@RecordingNotices.Moved(moved.Title)" Role="status" BaseClass="capture__notice"
+               TestId="@(string.IsNullOrWhiteSpace(TestId) ? null : $"{TestId}-moved")" />
+    }
 }
 
 @code {

src/App/Backlog.Mobile.UI/Recordings/FlatLevelDetector.cs

@@ -0,0 +1,39 @@
+namespace Backlog.Mobile.UI.Recordings;
+
+/// <summary>
+/// Notices a recording that hears nothing at all: a level of exactly 0 held for
+/// <see cref="Hold"/> while recording. A room is never that quiet — the microphone's own noise
+/// moves the level — so a level that stays at 0 means another app has the microphone, on a
+/// recorder that cannot say so itself (Android before 10, and the browser).
+/// </summary>
+/// <remarks>Fed from the recorder's own threads, so every reading is taken under a lock.</remarks>
+public sealed class FlatLevelDetector
+{
+    /// <summary>Ten seconds: long enough that a pause in speech never trips it.</summary>
+    public static readonly TimeSpan Hold = TimeSpan.FromSeconds(10);
+
+    private readonly Lock _gate = new();
+    private DateTimeOffset? _silentSince;
+
+    /// <summary>Takes one reading; answers whether the level has been 0 for <see cref="Hold"/>.</summary>
+    public bool Observe(double level, DateTimeOffset now)
+    {
+        lock (_gate)
+        {
+            if (level > 0)
+            {
+                _silentSince = null;
+                return false;
+            }
+
+            _silentSince ??= now;
+            return now - _silentSince.Value >= Hold;
+        }
+    }
+
+    /// <summary>Forgets the silence so far: a pause or a resume starts the count again.</summary>
+    public void Reset()
+    {
+        lock (_gate) _silentSince = null;
+    }
+}