Center login panel content, pin legal links to the bottom

Making the auth panel a fixed height (to kill the scrollbar) left the
shorter views (login, reset) stranded at the top with dead space below.
Split panel_content into a main_column (header, message, mode views -
expand-fill, center-aligned) and the legal footer as a sibling that
stays pinned to the panel's bottom edge regardless of which view is
showing or how tall its content is.

Verified headlessly: top/bottom whitespace above and below the active
view's content is now equal (e.g. login 174/174, steam 2/2), and the
footer's position doesn't move between modes.

scripts/ui/login_screen.gd

@@ -196,16 +196,27 @@ func _build_ui() -> void:
 	auth_panel.add_child(scroll)
 	panel_gutter = MarginContainer.new()
 	panel_gutter.size_flags_horizontal = Control.SIZE_EXPAND_FILL
+	panel_gutter.size_flags_vertical = Control.SIZE_EXPAND_FILL
 	panel_gutter.add_theme_constant_override("margin_right", 28)
 	scroll.add_child(panel_gutter)
 	panel_content = VBoxContainer.new()
 	panel_content.size_flags_horizontal = Control.SIZE_EXPAND_FILL
-	panel_content.add_theme_constant_override("separation", 16)
+	panel_content.size_flags_vertical = Control.SIZE_EXPAND_FILL
+	panel_content.add_theme_constant_override("separation", 24)
 	panel_gutter.add_child(panel_content)
 
+	# main_column holds everything except the legal footer and is the part
+	# that should center vertically when a view (e.g. login) is shorter than
+	# the panel - the footer stays pinned to the very bottom either way.
+	var main_column := VBoxContainer.new()
+	main_column.size_flags_vertical = Control.SIZE_EXPAND_FILL
+	main_column.alignment = BoxContainer.ALIGNMENT_CENTER
+	main_column.add_theme_constant_override("separation", 16)
+	panel_content.add_child(main_column)
+
 	var panel_header := HBoxContainer.new()
 	panel_header.add_theme_constant_override("separation", 18)
-	panel_content.add_child(panel_header)
+	main_column.add_child(panel_header)
 	var small_icon := TextureRect.new()
 	small_icon.texture = Brand.ranaze_mark_texture()
 	small_icon.custom_minimum_size = Vector2(72, 72)
@@ -229,14 +240,14 @@ func _build_ui() -> void:
 	reset_view = _build_reset_view()
 	steam_view = _build_steam_view()
 	for view in [login_view, register_view, reset_view, steam_view]:
-		panel_content.add_child(view)
+		main_column.add_child(view)
 
 	message_label = Label.new()
 	message_label.autowrap_mode = TextServer.AUTOWRAP_WORD_SMART
 	message_label.horizontal_alignment = HORIZONTAL_ALIGNMENT_CENTER
 	message_label.add_theme_font_size_override("font_size", 14)
 	message_label.add_theme_font_override("font", Brand.font)
-	panel_content.add_child(message_label)
+	main_column.add_child(message_label)
 	var legal_footer := HBoxContainer.new()
 	legal_footer.alignment = BoxContainer.ALIGNMENT_CENTER
 	legal_footer.add_theme_constant_override("separation", 12)