Refactor DM Zap button to Composer area

Moves the Zap button from the message thread header to the message composer area, adjacent to the Send button. Refactors the Zap interface creation logic into a standalone component `ZapInterface.js` to decouple it from `MessageThread.js`. Updates `Composer.js` to support an optional `onZap` handler and renders the button with an icon-only style matching the video player's accent interaction. Updates `AppShell.js` to orchestrate the Zap interface display.

js/ui/dm/AppShell.js

@@ -4,6 +4,7 @@ import { MessageThread } from "./MessageThread.js";
 import { NotificationCenter } from "./NotificationCenter.js";
 import { DMPrivacySettings } from "./DMPrivacySettings.js";
 import { aggregateZapTotals } from "./zapHelpers.js";
+import { createZapInterface } from "./ZapInterface.js";
 
 function createElement(doc, tag, className, text) {
   const element = doc.createElement(tag);
@@ -233,6 +234,29 @@ export class AppShell {
         zapConfig,
       }),
     );
+    const handleZapTrigger = () => {
+      const existingInterface = main.querySelector(".dm-zap-interface");
+      if (existingInterface) {
+        existingInterface.remove();
+        return;
+      }
+
+      const zapInterface = createZapInterface(doc, {
+        contact: activeConversation,
+        zapReceipts: activeZapReceipts,
+        onSendZap,
+        zapConfig
+      });
+
+      // Position it relative to the composer or just absolute in main
+      // The extracted createZapInterface uses 'absolute bottom-16 right-4' which positions it nicely
+      // relative to the container if the container is relative.
+      // main usually has relative positioning or we can append to root.
+      // AppShell root has absolute positioning context maybe?
+      // If main is the container, we append there.
+      main.appendChild(zapInterface);
+    };
+
     main.appendChild(
       Composer({
         document: doc,
@@ -242,8 +266,7 @@ export class AppShell {
         zapRecipient: activeConversation,
         zapConfig,
         onSend: onSendMessage,
-        // Zap button moved to MessageThread, so onSendZap here might be redundant if triggered from thread
-        // But Composer still handles text message sending.
+        onZap: handleZapTrigger,
       }),
     );
     main.appendChild(