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/chats.ts

@@ -1,5 +1,5 @@
 import { http } from "./http";
-import type { Chat, Message, MessageType } from "../chat/types";
+import type { Chat, ChatType, Message, MessageType } from "../chat/types";
 
 export async function getChats(): Promise<Chat[]> {
   const { data } = await http.get<Chat[]>("/chats");
@@ -7,10 +7,14 @@ export async function getChats(): Promise<Chat[]> {
 }
 
 export async function createPrivateChat(memberId: number): Promise<Chat> {
+  return createChat("private", null, [memberId]);
+}
+
+export async function createChat(type: ChatType, title: string | null, memberIds: number[] = []): Promise<Chat> {
   const { data } = await http.post<Chat>("/chats", {
-    type: "private",
-    title: null,
-    member_ids: [memberId]
+    type,
+    title,
+    member_ids: memberIds
   });
   return data;
 }