feat(wallet): unified GET /wallet/heights (tip per chain)

The wallet's ChainProvider seam fetches all chain tips in one call
(getBlockchainHeights) and selects per asset; the backend exposed per-chain
height endpoints but no aggregate, so the client's call 404'd. Add a
best-effort /wallet/heights returning {btc,ltc,xmr,wow,grin} concurrently — a
disabled or unreachable chain is null, never failing the others. JWT-gated,
OpenAPI-annotated (drift clean).

src/api/openapi.rs

@@ -58,6 +58,7 @@ use utoipa::OpenApi;
         crate::api::wallet::grin_relay::respond,
         crate::api::wallet::grin_relay::finalize,
         crate::api::wallet::grin_relay::cancel,
+        crate::api::wallet::heights::heights,
         crate::api::nip05::well_known_nostr,
     ),
     components(schemas(
@@ -132,6 +133,7 @@ use utoipa::OpenApi;
         crate::api::wallet::grin_relay::SlateIdRequest,
         crate::api::wallet::grin_relay::RelayEntry,
         crate::api::wallet::grin_relay::PendingRelaysResponse,
+        crate::api::wallet::heights::HeightsResponse,
         crate::api::nip05::WellKnownResponse,
     )),
     tags(