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/Plugin.cs

@@ -382,6 +382,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.SidebarModeAutoSwitchStep(this),
                 new SelfTests.ColorEditorBufferStep(this),
                 new SelfTests.ThemePickerCategoryStep(this),
+                new SelfTests.QuickPickerSelfTestStep(this),
                 new SelfTests.SettingsWindowOpenStep(this),
                 new SelfTests.OnOpenMainUiRoutesMainWindowStep(this),
                 new SelfTests.TypingIpcStateStep(this),