Mobile app cannot onboard without the desktop app — no standalone identity path

[suraciii]

Problem

The mobile app can't be used without first going through the desktop app.
The desktop app is a first-class client — it creates its own identity and
connects directly to a relay. The mobile app is not: it has no independent
identity path. It must receive an identity via NIP-AB pairing from a
desktop app that already has one.

This means anyone running the relay and agents on a headless server has no
way to onboard a mobile device. The mobile app already has the full client
surface — channels, threads, search, forum, agents, reactions, profiles —
but there is no path to reach it.

Workaround (what it takes today)

To get a phone connected without a desktop app, we had to:

1. Run buzz-pairing-cli source on the server to act as a headless
   pairing source.
2. Fix a payload format incompatibility — the CLI sends PayloadType::Nsec
   (raw nsec) but the mobile app expects PayloadType::Custom with
   {"relayUrl","pubkey","nsec"}, the format the desktop app produces.
3. Fix a rustls CryptoProvider panic (buzz-cli panics on agents draft-create because rustls CryptoProvider is not installed #2308) when connecting over wss://.
4. Copy-paste the nostrpair:// URI from a terminal to the phone.
5. Confirm the SAS code on both sides.

After these fixes, pairing succeeded and the mobile app connected. But this
is not a realistic path for users — it requires building from source,
patching the CLI, and juggling a terminal and a phone.

What would help

Two reasonable approaches, neither available today:

• Mobile creates its own identity (keypair) and connects directly, the
  same way the desktop app does.
• Mobile pairs with a headless identity source — the CLI already has a
  relay URL and nsec, but has no pair command to transfer them.

Either would make the mobile app usable without a desktop app.

Related: #2324 (pairing docs assume desktop), #2682 (same deadlock —
proposed a web client to avoid the desktop dependency entirely).