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/components/ChatList.tsx

@@ -1,4 +1,5 @@
 import { useChatStore } from "../store/chatStore";
+import { NewChatPanel } from "./NewChatPanel";
 
 export function ChatList() {
   const chats = useChatStore((s) => s.chats);
@@ -8,6 +9,7 @@ export function ChatList() {
   return (
     <aside className="w-full max-w-xs border-r border-slate-700 bg-panel">
       <div className="border-b border-slate-700 p-3 text-sm font-semibold">Chats</div>
+      <NewChatPanel />
       <div className="max-h-[calc(100vh-56px)] overflow-auto">
         {chats.map((chat) => (
           <button