feat(01-05): surface federation sync failures to the operator (FED-02)

A failed federation sync existed only as a `debug!` line on the node, so a
peer that had not synced in days looked identical in the UI to one that
synced a minute ago. Now the failure is persisted per peer and rendered.

- `FederatedNode.last_sync_error` / `.last_sync_error_at` — the failure-side
  mirror of the existing `last_transport` / `last_transport_at` pair.
- `federation::record_sync_result(data_dir, did, outcome)` — records the
  message on `Err`, CLEARS both fields on `Ok` so the badge disappears when
  the peer recovers. Runs under FEDERATION_STORE_LOCK via the `*_inner`
  load/save convention established by plan 01-01. An unknown DID is a silent
  Ok that writes nothing, so a peer removed mid-pass is never resurrected by
  an in-flight sync's error write. Skips the save entirely when nothing
  changed, keeping the steady state read-only rather than rewriting
  nodes.json (and contending for the lock) every 90s.
- Message truncated to MAX_SYNC_ERROR_CHARS (256), counted in chars not
  bytes so truncation cannot split a UTF-8 sequence (T-01-18).
- The 90s auto-sync loop calls it on both arms; the existing `debug!` line
  is kept — persisting is additive, not a replacement for logs.
- `federation.list-nodes` emits both fields when set, omits them when unset.
- NodeList renders a red SYNC badge beside the transport badge on both the
  trusted-node and peer rows, message + age in the `title` so the row stays
  single-line.

Tests (written first, confirmed failing — 16 compile errors, E0425 on
`record_sync_result` and E0609 on `last_sync_error`):
- persists_error / success_clears_error / missing_did_is_noop /
  on_empty_store_is_noop / truncates_long_error
- NodeList: badge present when set, ABSENT when unset (the guard against a
  badge that always renders), and present on an observer peer row.

cargo test -p archipelago federation — 42 passed, 0 failed.
vitest NodeList.test.ts — 4 passed. npm run build — green.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/federation/handlers.rs

@@ -262,6 +262,16 @@ impl RpcHandler {
                 if let Some(state) = &n.last_state {
                     obj["last_state"] = serde_json::to_value(state).unwrap_or_default();
                 }
+                // FED-02: surface the most recent sync failure so the operator
+                // sees a stale peer in the UI instead of it living only in the
+                // node's debug log. Omitted (not null) when the last attempt
+                // succeeded, so a recovered peer's badge disappears.
+                if let Some(err) = &n.last_sync_error {
+                    obj["last_sync_error"] = serde_json::json!(err);
+                }
+                if let Some(at) = &n.last_sync_error_at {
+                    obj["last_sync_error_at"] = serde_json::json!(at);
+                }
                 obj
             })
             .collect();
@@ -659,6 +669,8 @@ impl RpcHandler {
             fips_npub,
             last_transport: None,
             last_transport_at: None,
+            last_sync_error: None,
+            last_sync_error_at: None,
         };
 
         federation::add_node(&self.config.data_dir, node).await?;