Toast notifications now land bottom-center on mobile, and their icons are color-coded

Every toast notification in the dashboard (after creating or updating an agent, a site, a connector, or an organization setting) was pinned to the top-right corner regardless of screen size. On mobile, that meant notifications appeared at the top of the screen instead of the bottom, out of the way of one-handed use.

Toasts now default to top-right on desktop and bottom-center on mobile, matching the rest of the dashboard's responsive behavior.

The status icon on each toast is also color-coded now: success is brand green, info and warning are orange, and error is red. Previously every icon rendered in the same neutral gray, so you had to read the message to tell a success toast from an error.