Add username search and improve chat creation UX

Backend user search:

- Added users search endpoint for @username lookup.

- Implemented repository/service/router support with bounded result limits.

Web chat creation:

- Added API client for /users/search.

- Added NewChatPanel for creating private chats via @username search.

- Added group/channel creation flow from sidebar.

UX refinement:

- Hide message composer when no chat is selected.

- Show explicit placeholder: 'Выберите чат, чтобы начать переписку'.

- Added tsbuildinfo ignore rule.

web/src/api/users.ts

@@ -0,0 +1,9 @@
+import { http } from "./http";
+import type { UserSearchItem } from "../chat/types";
+
+export async function searchUsers(query: string, limit = 20): Promise<UserSearchItem[]> {
+  const { data } = await http.get<UserSearchItem[]>("/users/search", {
+    params: { query, limit }
+  });
+  return data;
+}