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.

app/users/service.py

@@ -16,6 +16,22 @@ async def get_user_by_username(db: AsyncSession, username: str) -> User | None:
     return await repository.get_user_by_username(db, username)
 
 
+async def search_users_by_username(
+    db: AsyncSession,
+    *,
+    query: str,
+    limit: int = 20,
+    exclude_user_id: int | None = None,
+) -> list[User]:
+    safe_limit = max(1, min(limit, 50))
+    return await repository.search_users_by_username(
+        db,
+        query=query,
+        limit=safe_limit,
+        exclude_user_id=exclude_user_id,
+    )
+
+
 async def update_user_profile(
     db: AsyncSession,
     user: User,