mobile: billing & usage in network settings (parity phase 5)

Surface the network plan, daily usage, and Stripe management — all backed
by client methods that already existed. New use-billing hooks and a
BillingSection (mirroring desktop): every member sees the plan + usage
summary; admins get cadence selection + "Upgrade to Pro" (checkout) and
"Manage subscription" (portal), opening Stripe in the system browser.
Added to NetworkSettingsScreen.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01LqGPzXQ1AA9CqYHgCgmbtV

js/mobile/src/features/network-settings/NetworkSettingsScreen.tsx

@@ -15,6 +15,7 @@ import { Avatar } from '@/components/Avatar';
 import { toUserMessage } from '@/lib/errors';
 import type { RootStackScreenProps } from '@/navigation/types';
 import { AddMembersSheet } from './AddMembersSheet';
+import { BillingSection } from './BillingSection';
 
 export function NetworkSettingsScreen({
   route,
@@ -157,6 +158,10 @@ export function NetworkSettingsScreen({
             )}
           </View>
         ) : null}
+
+        <View className="mt-4 border-t border-border pt-2">
+          <BillingSection networkId={networkId} />
+        </View>
       </ScrollView>
 
       {isAdmin ? (