feat(federation): cancel button for outbound pending peer requests

Previously the Pending Peer Requests panel only had Approve/Reject for
inbound rows; outbound rows in the 'sent' state had no action and
would sit there until the target explicitly approved or rejected. Now
you can Cancel an outbound request — the local row is dropped and a
PeerCancel nostr DM is sent so the target's inbound row also
disappears.

Backend:
- HandshakeMessage::PeerCancel {reason: Option<String>} variant.
- nostr_handshake::send_peer_cancel() mirrors send_peer_reject.
- handshake.poll handler dispatches inbound PeerCancel: finds the
  matching inbound pending row (same from_nostr_pubkey, state=Pending)
  and deletes it. Reply shape gains `cancelled_inbound: [id]`.
- federation::pending::delete() — hard-remove (set_state only
  transitions; we don't want 'Cancelled' ghosts in the audit trail).
- federation.cancel-request RPC: outbound+Sent only, default
  notify=true (cancelling silently is a footgun), best-effort DM
  (relay failure doesn't block local deletion). Wired in dispatcher.

Frontend:
- PendingRequestsPanel.vue: Cancel button appears only on
  outbound+sent rows. Emits 'cancel' event with request id.
- Federation.vue: cancelPending(id) handler calls
  rpcClient.federationCancelRequest and reloads the list.
- rpcClient.federationCancelRequest(id, reason?, notify=true).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

neode-ui/src/api/rpc-client.ts

@@ -742,6 +742,25 @@ class RPCClient {
     })
   }
 
+  /// Withdraw an outbound peer request we sent. Default notify=true so
+  /// the recipient's inbound row disappears from their UI (the main
+  /// UX reason to cancel — avoid leaving a stale handshake dangling).
+  async federationCancelRequest(
+    id: string,
+    reason?: string,
+    notify = true,
+  ): Promise<{ cancelled: boolean; id: string; notified: boolean }> {
+    return this.call({
+      method: 'federation.cancel-request',
+      params: {
+        id,
+        ...(reason ? { reason } : {}),
+        notify,
+      },
+      timeout: 30000,
+    })
+  }
+
   async federationSyncState(): Promise<{
     synced: number
     failed: number

neode-ui/src/views/Federation.vue

@@ -91,6 +91,7 @@
         @poll="pollHandshake"
         @approve="approvePending"
         @reject="rejectPending"
+        @cancel="cancelPending"
       />
 
       <NodeList
@@ -346,6 +347,22 @@ async function rejectPending(id: string) {
   }
 }
 
+async function cancelPending(id: string) {
+  pendingBusyId.value = id
+  discoveryError.value = ''
+  try {
+    // Default notify=true from the rpc-client — the peer's inbound row
+    // disappears from their UI so they don't have to wonder about a
+    // stale handshake.
+    await rpcClient.federationCancelRequest(id)
+    await loadPendingRequests()
+  } catch (e: unknown) {
+    discoveryError.value = e instanceof Error ? e.message : 'Cancel failed'
+  } finally {
+    pendingBusyId.value = null
+  }
+}
+
 function isOnlineCheck(node: FederatedNode): boolean {
   if (!node.last_seen) return false
   const lastSeen = new Date(node.last_seen).getTime()

neode-ui/src/views/federation/PendingRequestsPanel.vue

@@ -56,6 +56,16 @@
               Reject
             </button>
           </div>
+          <div v-else-if="req.outbound && req.state === 'sent'" class="flex flex-col gap-2 shrink-0">
+            <button
+              class="px-3 py-1 glass-button glass-button-sm rounded text-xs text-white/70 hover:text-red-300 disabled:opacity-50"
+              :disabled="busyId === req.id"
+              :title="'Withdraw the request and notify the peer to drop their pending row'"
+              @click="$emit('cancel', req.id)"
+            >
+              {{ busyId === req.id ? '…' : 'Cancel' }}
+            </button>
+          </div>
         </div>
       </div>
     </div>
@@ -76,6 +86,7 @@ defineEmits<{
   poll: []
   approve: [id: string]
   reject: [id: string]
+  cancel: [id: string]
 }>()
 
 // Hide already-handled rows older than 24h to keep the panel from growing