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/repository.py

@@ -1,4 +1,4 @@
-from sqlalchemy import select
+from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.users.models import User
@@ -31,3 +31,19 @@ async def list_users_by_ids(db: AsyncSession, user_ids: list[int]) -> list[User]
         return []
     result = await db.execute(select(User).where(User.id.in_(user_ids)))
     return list(result.scalars().all())
+
+
+async def search_users_by_username(
+    db: AsyncSession,
+    *,
+    query: str,
+    limit: int = 20,
+    exclude_user_id: int | None = None,
+) -> list[User]:
+    normalized = query.lower().strip().lstrip("@")
+    stmt = select(User).where(func.lower(User.username).like(f"%{normalized}%"))
+    if exclude_user_id is not None:
+        stmt = stmt.where(User.id != exclude_user_id)
+    stmt = stmt.order_by(User.username.asc()).limit(limit)
+    result = await db.execute(stmt)
+    return list(result.scalars().all())