Fix low-contrast session-expiry dialog in dark mode

The panel's shared Brand.panel_box background is ~80% opaque, which
compounded with the dialog's own dim backdrop let the screen behind
show through and washed out the card against it. Force full opacity
on this dialog's panel specifically, since other panel_box overlays
still rely on the shared translucency.

scripts/ui/session_expiry_dialog.gd

@@ -161,6 +161,7 @@ func _apply_theme() -> void:
 		return
 	var dark := UIThemeService.is_dark_mode()
 	var panel_style := Brand.panel_box(dark)
+	panel_style.bg_color.a = 1.0
 	panel_style.content_margin_left = 32
 	panel_style.content_margin_right = 32
 	panel_style.content_margin_top = 30