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/context/CompanyContext.tsx

@@ -10,6 +10,7 @@ import {
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import type { Company } from "@paperclip/shared";
 import { companiesApi } from "../api/companies";
+import { ApiError } from "../api/client";
 import { queryKeys } from "../lib/queryKeys";
 
 interface CompanyContextValue {
@@ -39,7 +40,17 @@ export function CompanyProvider({ children }: { children: ReactNode }) {
 
   const { data: companies = [], isLoading, error } = useQuery({
     queryKey: queryKeys.companies.all,
-    queryFn: () => companiesApi.list(),
+    queryFn: async () => {
+      try {
+        return await companiesApi.list();
+      } catch (err) {
+        if (err instanceof ApiError && err.status === 401) {
+          return [];
+        }
+        throw err;
+      }
+    },
+    retry: false,
   });
 
   // Auto-select first company when list loads