feat(ui): add auth pages, company rail, inbox redesign, and page improvements

Add Auth sign-in/sign-up page and InviteLanding page for invite acceptance.
Add CloudAccessGate that checks deployment mode and redirects to /auth when
session is required. Add CompanyRail with drag-and-drop company switching.
Add MarkdownBody prose renderer. Redesign Inbox with category filters and
inline join-request approval. Refactor AgentDetail to overview/configure/runs
views with claude-login support. Replace navigate() anti-patterns with <Link>
components in Dashboard and MetricCard. Add live-run indicators in sidebar
agents. Fix LiveUpdatesProvider cache key resolution for issue identifiers.
Add auth, health, and access API clients.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/components/BreadcrumbBar.tsx

@@ -33,7 +33,7 @@ export function BreadcrumbBar() {
   // Single breadcrumb = page title (uppercase)
   if (breadcrumbs.length === 1) {
     return (
-      <div className="border-b border-border px-4 md:px-6 py-4 flex items-center">
+      <div className="border-b border-border px-4 md:px-6 h-12 shrink-0 flex items-center">
         {menuButton}
         <h1 className="text-sm font-semibold uppercase tracking-wider">
           {breadcrumbs[0].label}
@@ -44,7 +44,7 @@ export function BreadcrumbBar() {
 
   // Multiple breadcrumbs = breadcrumb trail
   return (
-    <div className="border-b border-border px-4 md:px-6 py-3 flex items-center">
+    <div className="border-b border-border px-4 md:px-6 h-12 shrink-0 flex items-center">
       {menuButton}
       <Breadcrumb>
         <BreadcrumbList>