Support mouse click-to-position cursor in text input

When using a terminal emulator that supports mouse reporting (e.g., Ghostty), it would be great if clicking within the Claude Code text input area moved the cursor to that position. Currently, cursor repositioning is keyboard-only (arrow keys, Ctrl+A/E, Option+arrows). Many terminal apps (vim, tmux) already handle mouse events — would be a nice UX improvement for the input box.