style(modal): add subtle white ring in dark mode
Adds dark:ring-1 dark:ring-white/15 to the base Modal so every modal (Welcome,
Settings, MCP, etc.) reads as a defined surface against the dark backdrop.
Drops the redundant border/bg/radius classes from the settings layout's width
prop now that the base Modal owns them.
Also rounds the bottom corners of the WelcomeModal hero image (rounded-b-xl
overflow-hidden) so the GIF doesn't bleed past the modal edge.
