feat: rewrite the nip-4e implementation (#1)

Make NIP-4e a core feature, not an optional feature.

Note:
- The UI is broken and needs to be updated in a separate PR.

Reviewed-on: #1

crates/state/src/event.rs

@@ -0,0 +1,46 @@
+use std::collections::HashSet;
+use std::sync::{Arc, OnceLock};
+
+use nostr_sdk::prelude::*;
+use smol::lock::RwLock;
+
+static TRACKER: OnceLock<Arc<RwLock<EventTracker>>> = OnceLock::new();
+
+pub fn tracker() -> &'static Arc<RwLock<EventTracker>> {
+    TRACKER.get_or_init(|| Arc::new(RwLock::new(EventTracker::default())))
+}
+
+/// Event tracker
+#[derive(Debug, Clone, Default)]
+pub struct EventTracker {
+    /// Tracking events sent by Coop in the current session
+    sent_ids: HashSet<EventId>,
+
+    /// Events that need to be resent later
+    pending_resend: HashSet<(EventId, RelayUrl)>,
+}
+
+impl EventTracker {
+    /// Check if an event was sent by Coop in the current session.
+    pub fn is_sent_by_coop(&self, id: &EventId) -> bool {
+        self.sent_ids.contains(id)
+    }
+
+    /// Mark an event as sent by Coop.
+    pub fn sent(&mut self, id: EventId) {
+        self.sent_ids.insert(id);
+    }
+
+    /// Get all events that need to be resent later for a specific relay.
+    pub fn pending_resend(&mut self, relay: &RelayUrl) -> Vec<EventId> {
+        self.pending_resend
+            .extract_if(|(_id, url)| url == relay)
+            .map(|(id, _url)| id)
+            .collect()
+    }
+
+    /// Add an event (id and relay url) to the pending resend set.
+    pub fn add_to_pending(&mut self, id: EventId, url: RelayUrl) {
+        self.pending_resend.insert((id, url));
+    }
+}