Group DMs can never be renamed — no principal ever holds owner/admin on a DM channel, and community owner has no bypass

**Motivation**

Renaming a group DM is impossible for anyone, by construction:

- The relay creates DM channels under its own identity, and every participant is enrolled with the plain `member` role. No human ever gets `owner` or `admin` on a DM.
- The kind-41 handler (`crates/buzz-relay/src/handlers/side_effects.rs`) requires channel `owner`/`admin` for `name`/`about`/`archived`/`visibility`/`ttl` changes. The only escape hatch is being the NIP-OA owner of an owner-role agent in the channel, which a DM never has either.
- Community ownership doesn't help. There is no `RELAY_OWNER_PUBKEY` bypass in that code path, so even the community owner gets `actor not authorized for name/about/archived/visibility/ttl changes`.
- Desktop is consistent with this: `ChannelManagementSheet.tsx` gates the name editor on `channelType !== "dm"`, so the control never renders.

Net effect: a group DM is stuck displaying as "Group DM (N)" (or the participant list) forever. Ran into this on a self-hosted relay wanting to name a 3-person founders DM. The only working path I found was flipping my role to `owner` directly in Postgres, sending the kind-41 through the relay, then flipping it back. That obviously shouldn't be the supported path.

**Proposed solution**

Let DM participants rename their own DM. Slack's model here is the least surprising: any member of a group DM can set its name, and the name is scoped to that conversation. Concretely, in the kind-41 privileged-tag check, treat `channelType == "dm"` + actor is an active participant as sufficient for the `name` tag (the other privileged tags can stay locked down).

**Alternatives considered**

- Grant the DM creator `owner` at creation time. Works, but resurrects the orphaned-owner problem (#2928) when that identity goes away.
- A community-owner bypass for channel metadata. Broader than needed for this, and #2928 deliberately argues against widening community-owner authority over healthy channels.
- Desktop-side local aliases (client-only names). Avoids relay changes but every participant sees a different name, which defeats the point of naming a shared conversation.

**Additional context**

Related but distinct: #2928 (recovering orphaned channel ownership) and #4225 (owner_only add policy has no admin override). This one differs in that there is no owner to recover — DMs never have one.

Duplicate search: none found for DM renaming specifically.

Start with the kind-41 privileged-tag check in crates/buzz-relay/src/handlers/side_effects.rs, then inspect the DM name-editor gate in ChannelManagementSheet.tsx. Confirm that an active DM participant can change only the name and that the other privileged tags remain restricted; done means the shared DM name can be changed through the relay and the desktop editor is available.

Written by the indexing model from the issue text.