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/schemas.py

@@ -25,3 +25,11 @@ class UserRead(UserBase):
 class UserProfileUpdate(BaseModel):
     username: str | None = Field(default=None, min_length=3, max_length=50)
     avatar_url: str | None = Field(default=None, max_length=512)
+
+
+class UserSearchRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    username: str
+    avatar_url: str | None = None