Add IPC to prevent Signal session corruption from concurrent connections

When the relay is running, `warelay send` and `warelay heartbeat` now
communicate via Unix socket IPC (~/.warelay/relay.sock) to send messages
through the relay's existing WhatsApp connection.

Previously, these commands created new Baileys sockets that wrote to the
same auth state files, corrupting the Signal session ratchet and causing
the relay's subsequent sends to fail silently.

Changes:
- Add src/web/ipc.ts with Unix socket server/client
- Relay starts IPC server after connecting
- send command tries IPC first, falls back to direct
- heartbeat uses sendWithIpcFallback helper
- inbound.ts exposes sendMessage on listener object
- Messages sent via IPC are added to echo detection set

src/web/inbound.ts

@@ -212,6 +212,51 @@ export async function monitorWebInbox(options: {
       }
     },
     onClose,
+    /**
+     * Send a message through this connection's socket.
+     * Used by IPC to avoid creating new connections.
+     */
+    sendMessage: async (
+      to: string,
+      text: string,
+      mediaBuffer?: Buffer,
+      mediaType?: string,
+    ): Promise<{ messageId: string }> => {
+      const jid = `${to.replace(/^\+/, "")}@s.whatsapp.net`;
+      let payload: AnyMessageContent;
+      if (mediaBuffer && mediaType) {
+        if (mediaType.startsWith("image/")) {
+          payload = {
+            image: mediaBuffer,
+            caption: text || undefined,
+            mimetype: mediaType,
+          };
+        } else if (mediaType.startsWith("audio/")) {
+          payload = {
+            audio: mediaBuffer,
+            ptt: true,
+            mimetype: mediaType,
+          };
+        } else if (mediaType.startsWith("video/")) {
+          payload = {
+            video: mediaBuffer,
+            caption: text || undefined,
+            mimetype: mediaType,
+          };
+        } else {
+          payload = {
+            document: mediaBuffer,
+            fileName: "file",
+            caption: text || undefined,
+            mimetype: mediaType,
+          };
+        }
+      } else {
+        payload = { text };
+      }
+      const result = await sock.sendMessage(jid, payload);
+      return { messageId: result?.key?.id ?? "unknown" };
+    },
   } as const;
 }