test(postit): pin inoperative toolbar buttons (ACL, Mes cercles, [DEV] Signature) with headless UI tests

Three buttons on MainPage's toolbar are reported as inoperative
in the running app: ACL, Mes cercles, and [DEV] Signature. They
click but no dialog / page opens.

This commit adds headless UI tests that drive each button via
the Avalonia headless harness (KeyPressQwerty(Enter) on a
focused, x:Name'd button, per the CalculatorTests pattern in
Avalonia.Samples) and asserts the post-click top of
NavRoot.NavigationStack is a non-null Page.

The tests fail today on every button (stack size before == after
== 1): the click does not push anything. The bug is the user's
real complaint — the test is now wired to catch it.

To make the buttons reachable by the harness without walking
the visual tree (which does not see buttons hosted inside a
NavigationPage), name the two unnamed buttons:

- ACL  -> ManageAclButton
- Mes cercles -> OpenCirclesButton

([DEV] Signature was already named OpenSignatureDevButton.)

The XAML change is cosmetic; bindings and commands are
untouched. The test pattern follows SessionStatusBannerTests:
new MainWindow().Show(), PushAsync(MainPage), drive controls via
their generated x:Name fields.

src/PostIt/PostIt/Views/MainPage.axaml

@@ -33,8 +33,12 @@
                 <Button Command="{Binding Search}" Content="Filter" />
                 <Button Command="{Binding Save}" Content="Save" />
                 <Button Command="{Binding Delete}" Content="Delete" />
-                <Button Command="{Binding ManageAcl}" Content="ACL" />
-                <Button Command="{Binding OpenCircles}" Content="Mes cercles" />
+                <Button x:Name="ManageAclButton"
+                        Command="{Binding ManageAcl}"
+                        Content="ACL" />
+                <Button x:Name="OpenCirclesButton"
+                        Command="{Binding OpenCircles}"
+                        Content="Mes cercles" />
                 <!-- Publication toggle: a CheckBox wired to
                      DraftIsPublished. Clicking it fires
                      TogglePublishCommand, which pushes the