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/pages/ChatsPage.tsx

@@ -38,7 +38,13 @@ export function ChatsPage() {
         <div className="min-h-0 flex-1">
           <MessageList />
         </div>
-        <MessageComposer />
+        {activeChatId ? (
+          <MessageComposer />
+        ) : (
+          <div className="border-t border-slate-700 bg-panel p-4 text-center text-sm text-slate-400">
+            Выберите чат, чтобы начать переписку
+          </div>
+        )}
       </section>
     </main>
   );