Add keyboard shortcuts to research assistant input

[frano-m]

Summary

Add keyboard interaction support to the research assistant input field:

• Enter — submit the query
• Shift+Enter — insert a new line
• Up arrow — cycle through previous query history

Context

The assistant input currently submits via the form submit button. Adding keyboard shortcuts improves the conversational UX and matches standard chat input behavior.

Acceptance criteria

• Enter key submits the form
• Shift+Enter inserts a newline (input must support multiline)
• Up arrow key populates the input with the previous query from message history
• TypeScript compiles, lint passes, tests pass

[frano-m]

Complete