Phase 5: platform feature-flags CRUD, impersonation, billing catalogue

Three new platform screens on top of the Phase 1-4 work.

Feature flags (/feature-flags) — platform-wide flag registry. New route +
lib/arcadia/feature-flags.ts, capability platform.feature_flags, nav under
Automation. List/create/edit/delete with a per-row default toggle; pairs with
the Phase-4 per-tenant override tab.

Impersonation — "Impersonate" action on active users. Entirely client-side
token swap in session.ts (beginImpersonation parks the operator's session +
API token and swaps to the impersonation token; endImpersonation restores it),
with a sticky "Viewing as <email> — Stop" banner in the shell driven by the
JWT's impersonated_by claim. Stop is client-side because the impersonation
token carries the target's roles and can't reach the admin-gated /stop
endpoint; impersonation is stateless JWT so restoring the parked token is
sufficient.

Billing (/billing) — replaced the coming-soon stub with the real plan
catalogue from GET /billing/plans (lib/arcadia/billing.ts). Per-tenant plan
assignment stays on the tenant detail page; Entitlements + Apps remain honestly
marked "Soon".

Verified in-browser with real backend; typecheck adds zero errors (36→36).

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

app/lib/arcadia/billing.ts

@@ -0,0 +1,26 @@
+// Billing — the plan catalogue (what plans tenants can be on). Per-tenant plan
+// assignment lives on the tenant detail page; this is the platform-level view
+// of what's on offer. Backend: /api/v1/billing/plans.
+
+import type { ArcadiaClient } from "@crema/arcadia-core-client"
+
+export interface PlanMeter {
+  meter_key: string
+  included_units: number | null
+  overage_price_cents: number | null
+}
+
+export interface Plan {
+  name: string
+  slug: string
+  description: string | null
+  meters: PlanMeter[]
+  pricing: unknown[]
+  billing_track: string
+  trial_days: number
+}
+
+export async function listPlans(arcadia: ArcadiaClient): Promise<Plan[]> {
+  const res = await arcadia.GET<{ data: Plan[] }>("/api/v1/billing/plans")
+  return res.data
+}