Implement UI auto-detection and fallback logic

[itdove]

Description

Add smart auto-detection to choose between web UI and TUI based on environment.

Tasks

• Implement is_ssh_session() detection
• Implement has_browser() check
• Add DEVAIFLOW_PREFER_TUI env var support
• Implement should_use_web_ui() logic
• Add graceful fallback (web → TUI)
• Update daf config command with auto-detection
• Add --web and --tui explicit flags
• Add user feedback messages

Acceptance Criteria

• SSH sessions auto-select TUI
• Local sessions auto-select web UI
• DEVAIFLOW_PREFER_TUI=1 forces TUI
• --web flag forces web UI
• --tui flag forces TUI
• Web UI failure falls back to TUI gracefully
• Clear messages explain UI selection
• Both UIs remain fully functional

Estimated Effort: 2-3 days

[itdove]

Child of #305