Add insertTextAtCursorInternal() to properly handle newlines by splitting
lines at the cursor position.
- Normalize line endings (CRLF/CR to LF)
- Handle single-line and multi-line insertion
- Position cursor at end of inserted text
- Call onChange only once at the end
Refactor handlePaste() to use insertTextAtCursorInternal() for
multi-line pastes, while retaining character-by-character insertion for
single-line pastes to preserve autocomplete triggering.
7d2d170c1b