Give the application a settings area built from what really exists

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

@@ -249,12 +249,15 @@
               <Border Height="1" Background="{StaticResource BorderMid}" />
 
               <!--
-                Settings and Preferences both land on the one screen this application actually has — the
-                mock's own Settings area is a family of screens that do not exist yet; see the decided
-                deviation in design-notes/v5b-fidelity-notes.md. Two doors to one room rather than one door
-                removed, so neither word on the mock goes unanswered.
+                v5c: Settings, Vaults and Preferences now each land on their own page of the settings mode —
+                see MainWindowViewModel.EnterSettings and SettingsView.axaml. Settings opens on General, the
+                mode's own default landing page; Vaults and Preferences open directly on the page they name,
+                which is also what anything that used to navigate to ShellScreen.Vaults or
+                ShellScreen.Preferences now does — see ShowScreen. Three handlers rather than the two rows
+                sharing one before this wave: the mock's Settings area was a family of screens that did not
+                exist yet, and now that it does, "Settings" and "Preferences" are no longer the same click.
               -->
-              <Button Classes="poprow" Click="OnPopoverPreferencesPressed">
+              <Button Classes="poprow" Click="OnPopoverSettingsPressed">
                 <StackPanel Orientation="Horizontal" Spacing="10">
                   <TextBlock FontFamily="{StaticResource IconFont}" Text="&#xE8B8;" FontSize="12"
                              Foreground="{StaticResource Text}" />
@@ -282,8 +285,8 @@
 
               <!--
                 The existing sign-out flow, with its own confirm card — see
-                MainWindowViewModel.SignOutFromPopover for why this goes through Preferences rather than
-                calling SignOutCommand directly from wherever the popover happened to be opened.
+                MainWindowViewModel.SignOutFromPopover for why this goes through the Account settings page
+                rather than calling SignOutCommand directly from wherever the popover happened to be opened.
               -->
               <Button Classes="poprow" Click="OnPopoverLogoutPressed">
                 <StackPanel Orientation="Horizontal" Spacing="10">