feat(desktop): allow custom names for direct-message conversations

## Problem

Direct-message conversations are labeled from their participants and cannot be renamed. This is especially limiting for group DMs, where a participant list does not communicate the conversation’s purpose and becomes harder to scan as membership changes.

## Proposed behavior

Allow users to assign, edit, and clear a custom name for a DM conversation.

- Show a **Rename conversation** action in the DM header/menu.
- Display the custom name in the DM list and conversation header.
- When the custom name is cleared, fall back to the participant-derived label.
- Make the name shared consistently with all DM participants and across devices.
- Keep one-to-one DMs participant-named by default; custom naming can remain optional.

## Acceptance criteria

- A participant can set, change, and clear a custom DM name.
- All participants see the updated name without reopening the conversation.
- The custom name persists after restarting Buzz and on another signed-in device.
- Existing unnamed DMs continue to use their current participant-derived labels.
- The behavior is supported for group DMs and does not change channel naming.

## Current workaround

Create a private channel when a persistent named conversation is needed.

The issue names no files, tests, or code entry points; start by locating the DM header/menu, DM list, participant-derived labels, and the paths responsible for persistence and cross-device sharing. Done means participants can set, edit, and clear shared names for group DMs, with unnamed conversations retaining their current labels and channel naming unchanged.

Written by the indexing model from the issue text.