fix(desktop): explain the pairing 404 instead of surfacing a bare transport error - #5340

Closes #3779.

Duplicate check: none found for this change. #3779 has no PR attached. The related compose-side work (#2736, #3627, #3875, #4656) and the chart routing (#3424) fix the deployments; this fixes the message for every deployment they cannot reach — external reverse proxies, the Railway template mentioned in #3779, Helm installs without pairingRelay, and anything hand-rolled.

Problem

When a NIP-43 relay advertises no pairing_relay_url, pairing_relay_from_nip11 returns LegacyPath and resolve_pairing_relay_url appends /pair to the relay's own host. That address is chosen from an advertisement and never probed, so when nothing serves it the QR panel showed only:

WebSocket connection failed: HTTP error: 404 Not Found

No URL, no route decision, no config lever. Per #3779 and its confirmations, self-hosters had to read config.rs and pairing.rs to get unstuck.

Change

The route was already a typed enum but collapsed to a String before the task that reports failures could see it. PairingRelay is now Clone and travels into PairingTaskContext, so describe_connect_failure can explain the failure in terms of the decision that produced the URL.

A 404 on the legacy branch:

Pairing endpoint wss://relay.example.com/pair not found (404). This relay
advertises NIP-43 but no pairing_relay_url, so pairing fell back to
wss://relay.example.com/pair, and nothing is serving that path. The relay
operator needs to set BUZZ_PAIRING_RELAY_URL on the relay, or route /pair to a
buzz-pair-relay instance (see deploy/compose/README.md, "Device pairing").

Everything else keeps the original transport error and gains the attempted URL — the shape #3779 suggested, and what the split-domain case needs when a configured pairing_relay_url is simply wrong.

One deliberate narrowing: the actionable text is gated on 404 on the legacy branch specifically. A routed /pair whose sidecar is down answers 502, which is a different problem with a different fix, so it must not be told nothing is serving the path. There is a test for that.

Tests

Four added to pairing_relay_tests.rs, beside the existing route tests:

• legacy_path_404_explains_the_missing_pairing_endpoint — asserts the URL, the NIP-43 reason, and both levers appear
• legacy_path_non_404_keeps_the_transport_error_and_gains_the_url — the 502 case above
• configured_pairing_relay_404_reports_the_url_it_was_given — split domain; no /pair advice, which would point at the wrong file
• main_relay_failure_keeps_the_transport_error

cargo test -p buzz-desktop: 2274 passed, 0 failed. cargo fmt --check and cargo clippy --all-targets clean.

Why this one matters beyond the compose fix

We run a small managed Buzz fleet and hit this exact 404 on our own tenants (details in #2736). We had the source open, so we found it in an afternoon. The message is what everyone else has to work from, and it is the only part of this that helps a relay whose misconfiguration no upstream default can prevent.

The wording points at deploy/compose/README.md. If #2736 or a sibling lands and renames that section, this string should follow it — happy to rebase onto whichever lands first.