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>

neode-ui/src/views/federation/__tests__/NodeList.test.ts

@@ -29,4 +29,75 @@ describe('NodeList', () => {
     expect(wrapper.text()).toContain('Trusted Node')
     expect(wrapper.text()).not.toContain('Loading nodes...')
   })
+
+  // FED-02: a sync failure used to exist only as a debug! log line on the
+  // node, so a peer that hadn't synced in days looked identical to one that
+  // synced a minute ago. It must now be visible on the node's own row.
+  it('shows a sync-error badge on a node whose last sync failed', () => {
+    const failing: FederatedNode = {
+      ...trustedNode,
+      last_sync_error: 'Failed to reach federated peer',
+      last_sync_error_at: '2026-06-11T00:00:00Z',
+    }
+    const wrapper = mount(NodeList, {
+      props: {
+        nodes: [failing],
+        loading: false,
+        error: '',
+        syncResults: [],
+        dwnSyncDotClass: 'bg-green-400',
+        cleaningNodes: false,
+      },
+    })
+
+    const badge = wrapper.find('[data-testid="sync-error-badge"]')
+    expect(badge.exists()).toBe(true)
+    expect(badge.text()).toBe('SYNC')
+    // The full message + when it happened live in the tooltip, so the row
+    // stays single-line (the node-name truncate idiom).
+    expect(badge.attributes('title')).toContain('Failed to reach federated peer')
+  })
+
+  // The guard against a badge that always renders: a healthy peer (and a
+  // peer that has recovered, since record_sync_result clears the field) must
+  // show no sync-error badge at all.
+  it('renders no sync-error badge when last_sync_error is unset', () => {
+    const wrapper = mount(NodeList, {
+      props: {
+        nodes: [trustedNode],
+        loading: false,
+        error: '',
+        syncResults: [],
+        dwnSyncDotClass: 'bg-green-400',
+        cleaningNodes: false,
+      },
+    })
+
+    expect(wrapper.find('[data-testid="sync-error-badge"]').exists()).toBe(false)
+  })
+
+  // Peers (Observer level) render in a separate column — the badge must be
+  // on that row too, or a failing peer stays invisible.
+  it('shows the sync-error badge on an observer peer row', () => {
+    const peer: FederatedNode = {
+      ...trustedNode,
+      did: 'did:key:z6MkPeerNode',
+      trust_level: 'observer',
+      name: 'Peer Node',
+      last_sync_error: 'Peer returned 502 (via tor)',
+      last_sync_error_at: '2026-06-11T00:00:00Z',
+    }
+    const wrapper = mount(NodeList, {
+      props: {
+        nodes: [peer],
+        loading: false,
+        error: '',
+        syncResults: [],
+        dwnSyncDotClass: 'bg-green-400',
+        cleaningNodes: false,
+      },
+    })
+
+    expect(wrapper.find('[data-testid="sync-error-badge"]').exists()).toBe(true)
+  })
 })