tests: add regression coverage for chat-scoped skeleton replacement

[Senzo13]

Context

JoyBoy supports multiple conversations and background jobs. Skeleton/progress replacement must target the correct active chat, not an older .image-skeleton-message or .video-skeleton-message from another conversation.

Relevant files:

• web/static/js/chat.js
• web/static/js/generation.js
• web/static/js/app.js
• tests/

Suggested approach

Extract the DOM lookup logic for scoped skeleton replacement into a small testable helper, or add a lightweight JS/unit smoke test that builds a fake DOM with two chats and verifies the newest matching skeleton is updated.

Acceptance criteria

• A test covers two chat IDs with simultaneous/old skeletons.
• The helper chooses the skeleton matching the requested chatId.
• Legacy fallback still works only when no scoped skeleton exists.
• Existing Python tests and JS syntax checks still pass.