* fix: treat Option+Return as newline in multiline text inputs
Currently only Shift+Return inserts a newline in multiline text areas
(like the chat input). Option+Return (Alt+Return) was treated the same
as plain Return, triggering the send action.
Add altKey checks alongside the existing shiftKey checks in both:
- ChatInput: skip send when altKey is held
- PromptInputTextarea: early-return (allow newline) when altKey is held
* test: add alt-enter multiline regression coverage