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>

core/archipelago/src/api/rpc/handshake.rs

@@ -218,6 +218,7 @@ impl RpcHandler {
         let mut new_requests: Vec<PendingPeerRequest> = Vec::new();
         let mut applied_invites: Vec<String> = Vec::new();
         let mut rejected_outbound: Vec<String> = Vec::new();
+        let mut cancelled_inbound: Vec<String> = Vec::new();
         let mut skipped: Vec<String> = Vec::new();
 
         for hs in &handshakes {
@@ -354,6 +355,25 @@ impl RpcHandler {
                         );
                     }
                 }
+                HandshakeMessage::PeerCancel { reason } => {
+                    // Peer withdrew their PeerRequest — drop our matching
+                    // inbound pending row so it disappears from the UI.
+                    let pendings = pending::load_pending(&self.config.data_dir).await?;
+                    if let Some(row) = pendings.iter().find(|r| {
+                        !r.outbound
+                            && r.from_nostr_pubkey == hs.from_nostr_pubkey
+                            && matches!(r.state, PendingState::Pending)
+                    }) {
+                        let row_id = row.id.clone();
+                        pending::delete(&self.config.data_dir, &row_id).await?;
+                        cancelled_inbound.push(row_id);
+                        tracing::info!(
+                            from = %hs.from_nostr_pubkey,
+                            reason = ?reason,
+                            "Inbound peer request cancelled by sender"
+                        );
+                    }
+                }
             }
         }
 
@@ -362,6 +382,7 @@ impl RpcHandler {
             "new_requests": new_requests,
             "applied_invites": applied_invites,
             "rejected_outbound": rejected_outbound,
+            "cancelled_inbound": cancelled_inbound,
             "skipped": skipped,
         }))
     }