feat: add DM mode support to ChatHeader with recipient info

Adds dmMode option (hides member-list toggle), exposes hashEl in refs,
and adds updateChatHeaderForDm helper that swaps # for @, sets username
and status when a DM recipient is active.

Client/tauri-client/src/pages/main-page/ChatHeader.ts

@@ -3,7 +3,7 @@
  * and member-list toggle.
  */
 
-import { createElement, appendChildren } from "@lib/dom";
+import { createElement, appendChildren, setText } from "@lib/dom";
 import { createIcon } from "@lib/icons";
 
 // ---------------------------------------------------------------------------
@@ -11,6 +11,7 @@ import { createIcon } from "@lib/icons";
 // ---------------------------------------------------------------------------
 
 export interface ChatHeaderRefs {
+  readonly hashEl: HTMLSpanElement;
   readonly nameEl: HTMLSpanElement;
   readonly topicEl: HTMLSpanElement;
 }
@@ -19,6 +20,7 @@ export interface ChatHeaderOptions {
   readonly onTogglePins: () => void;
   readonly onToggleMembers: () => void;
   readonly onSearchFocus?: () => void;
+  readonly dmMode?: boolean;
 }
 
 // ---------------------------------------------------------------------------
@@ -64,8 +66,28 @@ export function buildChatHeader(
   });
   membersToggle.appendChild(createIcon("users", 18));
   membersToggle.addEventListener("click", () => opts.onToggleMembers());
+  if (opts.dmMode === true) {
+    membersToggle.style.display = "none";
+  }
   appendChildren(tools, searchInput, pinBtn, membersToggle);
 
   appendChildren(header, hash, nameEl, divider, topicEl, tools);
-  return { element: header, refs: { nameEl, topicEl } };
+  return { element: header, refs: { hashEl: hash, nameEl, topicEl } };
+}
+
+// ---------------------------------------------------------------------------
+// DM mode helper
+// ---------------------------------------------------------------------------
+
+export function updateChatHeaderForDm(
+  refs: ChatHeaderRefs,
+  recipient: { username: string; status: string } | null,
+): void {
+  if (recipient !== null) {
+    setText(refs.hashEl, "@");
+    setText(refs.nameEl, recipient.username);
+    setText(refs.topicEl, recipient.status);
+  } else {
+    setText(refs.hashEl, "#");
+  }
 }