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/router.py

@@ -4,8 +4,8 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.auth.service import get_current_user
 from app.database.session import get_db
 from app.users.models import User
-from app.users.schemas import UserProfileUpdate, UserRead
-from app.users.service import get_user_by_id, get_user_by_username, update_user_profile
+from app.users.schemas import UserProfileUpdate, UserRead, UserSearchRead
+from app.users.service import get_user_by_id, get_user_by_username, search_users_by_username, update_user_profile
 
 router = APIRouter(prefix="/users", tags=["users"])
 
@@ -15,6 +15,24 @@ async def read_me(current_user: User = Depends(get_current_user)) -> UserRead:
     return current_user
 
 
+@router.get("/search", response_model=list[UserSearchRead])
+async def search_users(
+    query: str,
+    limit: int = 20,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+) -> list[UserSearchRead]:
+    if len(query.strip().lstrip("@")) < 2:
+        return []
+    users = await search_users_by_username(
+        db,
+        query=query,
+        limit=limit,
+        exclude_user_id=current_user.id,
+    )
+    return users
+
+
 @router.get("/{user_id}", response_model=UserRead)
 async def read_user(user_id: int, db: AsyncSession = Depends(get_db), _current_user: User = Depends(get_current_user)) -> UserRead:
     user = await get_user_by_id(db, user_id)