[FEATURE] Build conversation input/output UI components

[UncleTallest]

Goal: Create the main conversation interface.

Tasks:

• Create input text entry widget
• Create output text view with scrolling
• Add send button
• Handle Enter key to send message
• Basic layout (vertical box with input at bottom)
• Test keyboard navigation

Success criteria:

• User can type messages and press Enter/click Send
• Output area scrolls properly
• UI follows GNOME HIG patterns

Output: src/claude_devel_client/ui/conversation.py