Sync: keep the remembered sign-in when the provider is merely down, and say why SYNC NOW found the machine offline

src/DodoSSH.Client.App/Views/SettingsPreferencesPage.axaml

@@ -87,6 +87,11 @@
                 <TextBlock Classes="settingsrowtitle" Text="Synchronise" />
                 <TextBlock Classes="settingsrowcaption"
                            Text="Runs a pass now. One runs on its own when the keychain opens, straight after any change, and every minute while it stays open." />
+                <!-- What SYNC NOW said. The status bar that carries it elsewhere is hidden in settings
+                     mode, so without this the button ran, failed or found the machine offline in silence. -->
+                <TextBlock Classes="settingsrowcaption"
+                           IsVisible="{Binding Vault.Status, Converter={x:Static StringConverters.IsNotNullOrEmpty}, FallbackValue=False}"
+                           Text="{Binding Vault.Status}" />
               </StackPanel>
               <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="8">
                 <Button Classes="ghost" Height="38" Content="SIGN IN" Command="{Binding SignInCommand}"

src/DodoSSH.Client.App/Views/SettingsVaultsPage.axaml

@@ -70,6 +70,13 @@
           </StackPanel>
         </Grid>
 
+        <!-- What SYNC NOW said. The status bar that carries it elsewhere is hidden in settings mode, so
+             without this the button ran, failed or found the machine offline in perfect silence. -->
+        <TextBlock Margin="0,10,0,0" FontSize="11.5" TextWrapping="Wrap" HorizontalAlignment="Right"
+                   Foreground="{StaticResource TextFaint}"
+                   IsVisible="{Binding Vault.Status, Converter={x:Static StringConverters.IsNotNullOrEmpty}, FallbackValue=False}"
+                   Text="{Binding Vault.Status}" />
+
         <!-- The name-a-vault and rename-a-vault forms, in place — this window has no modal idiom for a bare
              text field, and the members panel below is a different kind of overlay: over one vault's people,
              not over a form. -->

src/DodoSSH.Client.Shell/ViewModels/MainWindowViewModel.cs

@@ -3227,6 +3227,9 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
             if (token is null
                 || !Uri.TryCreate(vault.Session.Profile.ServerUrl, UriKind.Absolute, out var server))
             {
+                vault.OfflineReason = "This machine has no remembered sign-in. Press SIGN IN in "
+                    + "Preferences to start syncing.";
+
                 return null;
             }
 
@@ -3234,6 +3237,7 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
 
             connection = resumed;
             rememberedToken = token;
+            vault.OfflineReason = null;
 
             OnPropertyChanged(nameof(IsOnline));
             RaiseSyncState();
@@ -3244,15 +3248,18 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
 
             return resumed;
         }
-        catch (OidcException exception)
+        catch (OidcException exception) when (IsRefusedGrant(exception))
         {
             // The provider answered and said no: the session was revoked, or the token was rotated and
             // this machine kept the old one. Retrying costs a round trip a minute and can only ever get
             // the same answer, so the token goes and the user is told the one thing that fixes it.
             await ForgetSignInAsync(cancellationToken).ConfigureAwait(true);
 
-            Announce($"Your sign-in has expired, so this machine is offline: {exception.Message} "
-                + "Sign in again from Preferences to start syncing.");
+            var message = $"Your sign-in has expired, so this machine is offline: {exception.Message} "
+                + "Sign in again from Preferences to start syncing.";
+
+            vault.OfflineReason = message;
+            Announce(message);
 
             return null;
         }
@@ -3260,11 +3267,29 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
         {
             // Everything else is a machine with no network, a server that is down, or a vault that was
             // locked mid-attempt — all of which are ordinary and all of which resolve themselves. The
-            // titlebar already says OFFLINE; a socket error once a minute would say nothing more.
+            // titlebar already says OFFLINE; a socket error once a minute would say nothing more. That
+            // includes an OidcException with no refusal in it: discovery answering 502, or a token endpoint
+            // whose body is a proxy's error page, is the identity provider restarting. Forgetting the token
+            // for that used to strand a machine offline for good once the provider came back.
+            vault.OfflineReason = $"The server could not be reached: {exception.Message} Changes are "
+                + "queued and will be sent once it can.";
+
             return null;
         }
     }
 
+    /// <summary>
+    /// Whether a failed resume was the provider refusing the token, rather than failing to answer.
+    /// </summary>
+    /// <remarks>
+    /// <c>invalid_grant</c> is what RFC 6749 §5.2 has a token endpoint say about a refresh token that is
+    /// expired, revoked or already rotated — the one answer that a retry cannot change. Every other
+    /// <see cref="OidcException"/> on this path is discovery or the token endpoint failing, which is a
+    /// provider that is down and will be back.
+    /// </remarks>
+    private static bool IsRefusedGrant(OidcException exception) =>
+        string.Equals(exception.ErrorCode, "invalid_grant", StringComparison.Ordinal);
+
     /// <summary>
     /// Writes the connection's current refresh token into the vault, if it has changed.
     /// </summary>

src/DodoSSH.Client.Shell/ViewModels/VaultViewModel.cs

@@ -6199,8 +6199,10 @@ internal sealed partial class VaultViewModel(
                 if (await ResolveServerAsync(cancellationToken).ConfigureAwait(true) is not { } server)
                 {
                     LastSyncFailed = true;
-                    Status = "Offline. Changes are queued and will be sent as soon as this machine "
-                        + "can reach the server again.";
+                    Status = OfflineReason is { } reason
+                        ? $"Offline. {reason}"
+                        : "Offline. Changes are queued and will be sent as soon as this machine "
+                            + "can reach the server again.";
                     return;
                 }
 
@@ -6214,6 +6216,14 @@ internal sealed partial class VaultViewModel(
             }).ConfigureAwait(true);
     }
 
+    /// <summary>Why the last attempt to get online failed, or null when nothing has said.</summary>
+    /// <remarks>
+    /// Set by the shell, which is the thing that knows: <see cref="ServerReconnectHandler" /> answers null
+    /// for every reason on purpose, so that a background pass stays quiet. A press of SYNC NOW is somebody
+    /// asking, though, and "offline" with no why is what left a machine stuck with no way to find out.
+    /// </remarks>
+    internal string? OfflineReason { get; set; }
+
     /// <summary>
     /// Finds a server to sync against, getting this machine online if it is not.
     /// </summary>