No sitewide box-sizing: border-box reset exists, so .ui-dialog's
padding + border were adding on top of width: 90vw instead of being
carved out of it -- 401px rendered against a 390px viewport, real
horizontal overflow. Caught via a real 390x844 screenshot per mara's
"phone size?" ask on the PR, not assumed.
Adds a generic ui/dialog/Dialog primitive (native <dialog>, no
third-party modal lib and no shadow-DOM custom element per the esbuild
gap on those) and wires a "+ agent" button into AgentsPage that opens
the existing create-agent form inside it, content unchanged from its
former life as a standalone /create-agent route/nav item.
Removes the "new agent" top-level nav entry and the /create-agent
route entirely -- creation now only reachable from the roster that
gets populated by it. CreateAgentPage.tsx/css renamed to
CreateAgentForm.tsx/css to match its new role as a mounted component
rather than a page.
Screenshot-verified the dialog open/closed states against a mock
server.
