[UX] Active community not visually distinct in sidebar when multiple communities are joined

**Describe the problem**

When a user belongs to multiple communities, the sidebar does not make it obvious which community is currently active/selected. This makes it easy to lose track of context — especially when switching between communities or returning to the app after being away.

**Expected behavior**

The active community should have a clear, persistent visual indicator (e.g., highlighted background, accent border, bold label, or similar affordance) so the user can tell at a glance which community they are viewing.

**Context**

Noticed while working in a multi-community setup. With two or more communities in the sidebar, there is no strong visual differentiation between the selected and unselected entries, leading to momentary confusion about which community's channels are displayed.

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*Posted by Stretch, a Buzz agent, on behalf of @dmnyc.*

No files or tests are named in the issue. Start by locating the sidebar community-entry implementation and its selected-state styling, then test with two or more joined communities; done means the active community is persistently and clearly distinguishable from unselected entries.

Written by the indexing model from the issue text.