Admin UX overhaul: P0 fixes, error-UX foundations, nav cleanup, tenant detail page

From the 2026-07-14 UI/UX audit (17/40). Four phases:

P1 — Settings route crashed on Agents→Edit (Input/Textarea used but never
imported). Added imports + a shared route-level error boundary
(components/route-error.tsx) re-exported from all shell routes, so one
crashing panel degrades to an explained card with the nav intact instead of
replacing the whole app with a stack trace. Corrected the stale CLAUDE.md
claim that `npm run typecheck` crashes — it works, and would have caught the
missing import.

P2 — Error/empty/feedback foundations. Fixed useSession identity churn that
fired ~3x duplicate fetches per screen and self-inflicted 429s (referentially
stable snapshot). New lib/errors.ts (describeError → plain-language + the fix)
and components/data-state.tsx (DataState renders exactly one of
error/loading/empty/content, so a failed load never shows as "empty";
DialogError for in-dialog failures; 429 auto-retry). Rolled across all 15
list routes; every mutation now toasts. Surfaced+fixed two silent-failure
bugs (sso + buckets-CORS swallowed load errors; the latter could wipe rules
on save).

P3 — Nav IA + trust cleanup. Default-expanded rail; regrouped into 7 coherent
sections; collapsed the Apps/Plan/Entitlements stub triplication into one
honest /billing page; deleted fabricated seeded notifications and the dead
Help menu item; fixed the 403 copy (referenced a tenant switcher that doesn't
exist); removed orphan /assistant + /library routes; gated dev-seed login
hints behind DEV; renamed /activity → /audit-log with a redirect.

P4 — Tenant detail page (routes/tenants.$id.tsx + components/tenant-detail/*),
closing the provision→configure gap. 8 tabs (Overview, Plan & quotas,
Branding, Localization, Email & SMS, Feature flags, IP rules, Inbound
webhooks), each verified saving to the real backend. Row name links to detail.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/routes/ai.tsx

@@ -2371,3 +2371,5 @@ function VoiceInputButton({
     </button>
   )
 }
+
+export { RouteErrorBoundary as ErrorBoundary } from "~/components/route-error"