Account settings popover: "Add a community" is nested under the active community's submenu, reads as community-specific

## Summary

The account popover (opened by clicking your own name/avatar at the bottom of the sidebar) mixes global, account-level actions with actions scoped to whichever community is currently active, without a clear visual separation between them.

## Current behavior

Clicking the profile avatar opens a popover with:
- `Update your status` (global)
- `` row → expands to `Copy community URL`, `Community settings`, `Leave community` (all specific to that one community)
- `Add a community` (global — meant to add an *additional*, unrelated community to the account)
- `Send feedback`, `Settings` (global)

`Add a community` is rendered by the same `CommunitySwitcher` component as the community-specific actions (`Copy community URL`, `Leave community`, etc.), just not visually separated from them. With only one community in the account, there's no separate "switcher" section to make its global nature obvious — it collapses into looking like it belongs to whichever community is currently active.

## Why it's confusing

A user with one community sees "Add a community" sitting directly under (or alongside) that community's name and community-specific actions like "Leave community." It reads as "an action related to this community" rather than "add a completely separate, unrelated community to my account" — which is what it actually does.

## Suggested direction (not prescribing the fix)

Visually separate account-level actions (`Update your status`, `Add a community`, `Send feedback`, `Settings`) from the per-community submenu (`Copy community URL`, `Community settings`, `Leave community`) — e.g. a divider, distinct section header, or moving `Add a community` to sit with the other global items rather than adjacent to the community-specific ones. Needs a product/design decision on the right placement, not just a code shuffle — flagging for maintainer judgment.

## Where it lives

The popover in question is rendered by the sidebar's `CommunitySwitcher` component (`desktop/src/features/sidebar/`), triggered from the account/profile row at the bottom of the sidebar.