fix(shell): dark DockButton theme variation for the system dock

DarkPanel is a PanelContainer-base variation (only a `panel`
stylebox); a Button looks up normal/hover/pressed, which don't exist
on it, so SystemDock's menu buttons rendered as default gray Godot
buttons instead of mock-faithful dark nav buttons.

Add a dedicated Button-base DockButton variation (Palette-only
colours, dock-dark normal/hover/pressed styleboxes), point
system_dock.gd's menu buttons at it, and regenerate the committed
game_theme.tres artifact from the builder.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/scripts/ui/shell/system_dock.gd

@@ -32,7 +32,7 @@ func _ready() -> void:
 		var id: StringName = entry[0]
 		var button := Button.new()
 		button.text = "%s   %s" % [entry[2], entry[1]]
-		button.theme_type_variation = ThemeKeys.DARK_PANEL
+		button.theme_type_variation = ThemeKeys.DOCK_BUTTON
 		button.pressed.connect(func(): screen_requested.emit(id))
 		_menu.add_child(button)
 		_screen_buttons[id] = button