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/chat/types.ts

@@ -33,3 +33,9 @@ export interface TokenPair {
   refresh_token: string;
   token_type: string;
 }
+
+export interface UserSearchItem {
+  id: number;
+  username: string;
+  avatar_url: string | null;
+}