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/Components/InputBar.cs

@@ -40,6 +40,11 @@ internal sealed class InputBar
     private readonly Action _onOpenSettings;
     private readonly CommandHelpWindow _commandHelpWindow;
 
+    // Null in pop-out windows: the theme/tab quick-picker only belongs in the
+    // main window (1.5.4 had no pop-outs, and a tab jump from a channel-bound
+    // pop-out would be confusing). The main window's InputBar gets the instance.
+    private readonly ThemeQuickPicker? _themeQuickPicker;
+
     private string _pendingMessage = string.Empty;
     private bool _isFocused;
     private bool _wasInputTextHovered;
@@ -75,7 +80,8 @@ internal sealed class InputBar
         TokenResolver resolver,
         ILogger<InputBar> logger,
         Action onOpenSettings,
-        CommandHelpWindow commandHelpWindow
+        CommandHelpWindow commandHelpWindow,
+        ThemeQuickPicker? themeQuickPicker = null
     )
     {
         _symbolPicker = symbolPicker;
@@ -85,6 +91,7 @@ internal sealed class InputBar
         _logger = logger;
         _onOpenSettings = onOpenSettings;
         _commandHelpWindow = commandHelpWindow;
+        _themeQuickPicker = themeQuickPicker;
     }
 
     public string PendingMessage => _pendingMessage;
@@ -188,6 +195,9 @@ internal sealed class InputBar
         if (inserted is not null && _pendingMessage.Length + inserted.Length <= BufferCapacity)
             _pendingMessage += inserted;
 
+        // Theme/tab quick-picker popup (main window only; null in pop-outs).
+        _themeQuickPicker?.Draw();
+
         // Auto-translate popup runs after all other popups so the OpenPopup
         // anchor lands on the InputText item we just drew.
         DrawAutoCompletePopup();
@@ -507,6 +517,18 @@ internal sealed class InputBar
                     ImGui.SetTooltip("Insert symbol");
             }
 
+            if (_themeQuickPicker is not null)
+            {
+                ImGui.SameLine();
+                if (ImGui.Button(FontAwesomeIcon.Palette.ToIconString()))
+                    _themeQuickPicker.OpenPopup();
+                if (ImGui.IsItemHovered())
+                {
+                    using (ImRaii.DefaultFont())
+                        ImGui.SetTooltip(HellionStrings.Settings_QuickPicker_Tooltip);
+                }
+            }
+
             ImGui.SameLine();
             if (ImGui.Button(FontAwesomeIcon.Cog.ToIconString()))
             {