refactor: code health improvements for core modules

[aroff]

Summary

• Extract helper functions to reduce complexity in handlers.rs, server.rs, terminal.rs, and websocket.rs
• Consolidate error handling and response formatting patterns
• Separate data preparation from rendering logic
• Improve code organization and testability

Key Improvements

handlers.rs

• Consolidate error handling with print_ask_error and print_auth_result helpers
• Add generic prompt_with_validation helper for config prompts
• Extract parse_log_level function
• Reduce code duplication across authorization handlers

server.rs

• Extract process_websocket_message and handle_text_message methods
• Create dedicated message handler methods for questions, authorization, and navigation
• Improve separation of concerns in WebSocket message processing

terminal.rs

• Extract draw_terminal, fetch_channel_messages, and calculate_scroll_offset methods
• Separate data preparation from rendering with dedicated methods
• Improve modularity with build_message_line helper

websocket.rs

• Split buffer flushing into ensure_connection and send_buffered_messages
• Extract send_websocket_message and calculate_timeout helpers
• Create await_response_or_timeout with ResponseAwaitResult enum for clearer async flow

Test plan

• All existing tests pass
• Code formatting passes (cargo fmt)
• Linting passes (cargo check)
• No behavior changes - pure refactoring

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