Support multi-line query input with Shift+Enter

[rohitgs28]

Summary

Currently the query input is a single line. Complex queries need multi-line editing.

Expected Behavior

• Shift+Enter inserts a newline in the query input
• Enter (without Shift) still submits the query
• Input box expands vertically (up to a max height) to show multiple lines
• Cursor navigation works across lines (Up/Down move between lines when in multi-line mode)

Technical Notes

• The input is currently a String with a single cursor position
• Will need to track row/col position instead of flat index
• Rendering needs to handle line wrapping in the input Paragraph widget
• See src/app.rs KeyCode::Enter handling and src/ui/renderer.rs render_chat_panel()