Refactor DM Zap button and stats to Composer area

Moves the Zap button and Zap statistics 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 `zapStats` prop, rendering the button with an icon-only style and the stats pill reusing the existing CSS class. Updates `AppShell.js` to orchestrate the Zap interface display and pass the necessary statistics.

js/ui/dm/MessageThread.js

@@ -1,6 +1,5 @@
 import { DayDivider } from "./DayDivider.js";
 import { MessageBubble } from "./MessageBubble.js";
-import { formatZapAmount } from "./zapHelpers.js";
 
 function createElement(doc, tag, className, text) {
   const element = doc.createElement(tag);
@@ -76,28 +75,6 @@ export function MessageThread({
   privacyBadge.title = privacyHint;
   header.appendChild(privacyBadge);
 
-  const zapSummary = createElement(doc, "div", "dm-message-thread__zap-summary");
-  zapSummary.appendChild(
-    createElement(doc, "span", "dm-message-thread__zap-label", "Zaps"),
-  );
-  zapSummary.appendChild(
-    createElement(
-      doc,
-      "span",
-      "dm-message-thread__zap-total",
-      formatZapAmount(conversationZapTotalSats),
-    ),
-  );
-  zapSummary.appendChild(
-    createElement(
-      doc,
-      "span",
-      "dm-message-thread__zap-profile",
-      `Profile: ${formatZapAmount(profileZapTotalSats)}`,
-    ),
-  );
-  header.appendChild(zapSummary);
-
   if (typeof onMarkRead === "function") {
     const actions = createElement(doc, "div", "ml-auto flex items-center gap-2");
     const markReadButton = createElement(doc, "button", "btn-ghost", "Mark read");