[Feature] Reorder and color-code communities in sidebar

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

## Problem

When a user joins multiple communities, the sidebar lists them in a fixed order with no way to customize it. There is also no visual differentiation between communities beyond their name and icon, making it difficult to distinguish them at a glance — especially in combination with the active-community highlight issue (#2570), where the currently selected community is already hard to identify.

## Proposed solution

**1. Drag-to-reorder communities**

Allow users to reorder their community list via drag-and-drop (or up/down controls). The custom order should persist across sessions and devices (synced via account settings or local storage as a fallback).

**2. Color-coded community labels**

Give each community a user-selectable color tag — either a small colored dot/badge next to the community name, or a colored accent on the sidebar item. Users pick from a preset palette (or a custom hex color). This helps visually separate communities at a glance and pairs well with a fix for the active highlight state.

## Related

- #2570 — Active community not visually distinct in sidebar when multiple communities are joined

These two improvements are complementary: color-coding gives each community a persistent visual identity, and a clearer active highlight (per #2570) makes the selection state obvious against that identity.

The issue names no files, tests, or entry points. Start by locating the sidebar community-list implementation and the existing account-settings or local-storage persistence path. Done means communities can be reordered, each has a selectable color, and both preferences persist across sessions and devices where supported.

Written by the indexing model from the issue text.