feat(themes): restore the header theme/tab quick-picker

Brings back the 1.5.4 quick-picker lost in the v1.6.0 rewrite: a palette button
in the input-bar button row (left of the cog) opens a popup that switches the
theme (built-in + custom, active row checked) and jumps between chat tabs without
opening settings. Theme switch mirrors the settings ThemePicker; the tab jump
routes through a new MainWindow.ActivateTab that replays the click path
(previous -> set -> OnTabActivated) so tell/unread handling is unchanged. Main
window only -- pop-out InputBars get a null picker. Adds QuickPickerSelfTestStep.

HellionChat/Ui/Windows/MainWindow.cs

@@ -135,6 +135,19 @@ internal sealed class MainWindow : Window
             TabLifecycleHelpers.OnTabActivated(next, removed);
     }
 
+    // Programmatic tab activation for the header quick-picker. Mirrors the click
+    // path in TopTabBar/Sidebar exactly (previous → set → OnTabActivated) so a
+    // header pick strips tell-state and resets unread the way a real click does.
+    internal void ActivateTab(Tab tab)
+    {
+        if (ReferenceEquals(_activeTab, tab))
+            return;
+
+        var previous = _activeTab;
+        _activeTab = tab;
+        TabLifecycleHelpers.OnTabActivated(tab, previous);
+    }
+
     // Internal accessors for self-tests so the probes can reach the live
     // component without exposing them as public surface.
     internal Components.Sidebar GetSidebarForSelfTest() => _sidebar;