Remove dead "network" UI and code after Auth Service network removal

## Summary

**App:** `src/nexus/` in this repo. Paths below (which include their own `src/...`) are relative to that root — e.g. the first file is `src/nexus/src/app/(dashboard)/system/users/[id]/page.tsx`.

The Auth Service has fully removed the legacy "network" org-membership concept in favor of groups. Nothing crashes in Nexus as a result, but two things are now permanently dead and should be cleaned up.

## 1. "Current Access" panel will permanently show "No network roles assigned"

`src/app/(dashboard)/system/users/[id]/page.tsx` and `src/app/(dashboard)/system/team-members/[memberId]/page.tsx` read `user.networks` from `GET /users/:id` to render the "Networks" section of a "Current Access" panel. That field is no longer returned by the Auth Service, so the section will now permanently render its empty state.

**Fix:** remove the "Networks" section from both pages' "Current Access" panel.

## 2. Dead exports in `src/shared/hooks/useRBAC.ts`

`hasPermissionInNetwork`, `hasRoleInNetwork`, and `getUserNetworks` have zero call sites anywhere in Nexus (confirmed via a full audit).

**Fix:** delete these three exports whenever convenient.

## Priority

Not urgent — backlog cleanup, no user-facing errors or broken functionality.