Dock Back To Windows Button Display

Summary

The Dock "Back to platform" action should expose a configurable left icon slot while keeping the localized platform text fixed.

The default display mode is IconAndText so existing users keep a familiar Dock layout after upgrade.
The localized platform text remains controlled by the app and is not user-editable.
General > Basic Settings exposes one Fluent Avalonia FASettingsExpander for the back-to-platform button, with icon-related controls folded into nested FASettingsExpanderItem rows.
The main row exposes a dropdown with IconAndText, IconOnly, and TextOnly options.
A nested icon source row selects Fluent icon or text icon.
Fluent icon mode uses a popup picker-style flyout with search and a grid of the full FluentIcons Icon enum.
Text icon mode lets the user enter short text for the left icon slot.
Changing the dropdown persists to AppSettingsSnapshot.BackToWindowsButtonDisplayMode and updates the Dock button without restarting.
Changing the icon source, Fluent icon, or text icon persists to app settings and updates the Dock button without restarting.
IconOnly keeps the existing tooltip text so the button remains understandable.
PinnedTaskbarActions continues to control whether the action is visible; it does not replace the display mode setting.

With default settings, the Dock button shows a small circle icon and the localized platform text.
Selecting icon only hides the platform text and keeps the configured left icon visible.
Selecting text only hides the left icon slot and keeps the localized platform text visible.
Choosing a Fluent icon changes the left icon slot.
Entering a short text icon changes the left icon slot.
Restarting the app restores the selected display mode.
Clicking the button still runs the existing minimize/back-to-platform behavior.