Center the legal consent row instead of left-aligning it

Wrapped the row in a CenterContainer so the checkbox+text sit centered
under the field column instead of hugging the left edge. Had to drop
the RichTextLabel's EXPAND_FILL (meaningless inside a CenterContainer,
which never stretches its child) in favor of an explicit 480px
minimum width - without one, the label collapsed toward zero width
and wrapped character-by-character, which is what the first attempt
at this actually rendered as before I caught it.

Verified with a direct coordinate check: the row's horizontal center
now matches the confirm-password field's center exactly (delta 0.0).

scripts/ui/login_screen.gd

@@ -364,13 +364,14 @@ func _add_legal_consent(parent: VBoxContainer) -> CheckBox:
 	parent.add_child(check)
 
 	var margin := MarginContainer.new()
-	margin.add_theme_constant_override("margin_left", 6)
 	margin.add_theme_constant_override("margin_top", 8)
 	margin.add_theme_constant_override("margin_bottom", 8)
 	parent.add_child(margin)
+	var center := CenterContainer.new()
+	margin.add_child(center)
 	var row := HBoxContainer.new()
 	row.add_theme_constant_override("separation", 12)
-	margin.add_child(row)
+	center.add_child(row)
 
 	var square := Panel.new()
 	square.custom_minimum_size = Vector2(20, 20)
@@ -391,7 +392,7 @@ func _add_legal_consent(parent: VBoxContainer) -> CheckBox:
 	consent_text.fit_content = true
 	consent_text.scroll_active = false
 	consent_text.meta_underlined = false
-	consent_text.size_flags_horizontal = Control.SIZE_EXPAND_FILL
+	consent_text.custom_minimum_size = Vector2(480, 0)
 	consent_text.size_flags_vertical = Control.SIZE_SHRINK_CENTER
 	consent_text.add_theme_font_override("normal_font", Brand.font)
 	consent_text.add_theme_font_size_override("normal_font_size", 13)