Add WhatsApp reactions support

Summary:

Test Plan:

src/web/inbound.ts

@@ -566,6 +566,30 @@ export async function monitorWebInbox(options: {
       const jid = toWhatsappJid(to);
       await sock.sendPresenceUpdate("composing", jid);
     },
+    /**
+     * Send a reaction (emoji) to a specific message.
+     * Pass an empty string for emoji to remove the reaction.
+     */
+    sendReaction: async (
+      chatJid: string,
+      messageId: string,
+      emoji: string,
+      fromMe: boolean,
+      participant?: string,
+    ): Promise<void> => {
+      const jid = toWhatsappJid(chatJid);
+      await sock.sendMessage(jid, {
+        react: {
+          text: emoji,
+          key: {
+            remoteJid: jid,
+            id: messageId,
+            fromMe,
+            participant,
+          },
+        },
+      });
+    },
   } as const;
 }