feat: add payload copy, centralize UI theme, and extract UseCase interface

[xvlet]

- UI: Created theme.go to centralize lipgloss color definitions into an AppTheme struct.
- UI: Replaced hardcoded hex colors with AppTheme references across all TUI views and components.
- UI: Added c and y keybindings in the message detail view to copy the raw payload directly to the OS clipboard.
- UI: Updated the footer hints to display the new copy shortcut.
- UI: Implemented a success notification for clipboard actions and prevented these notifications from falsely triggering the disconnected status.
- UI: Updated AppModel to depend on the UseCase interface instead of a concrete struct pointer, strictly adhering to Clean Architecture principles.
- Usecase: Extracted UseCase interface to decouple the UI layer from concrete business logic implementations.
- Usecase: Renamed ActiveMQUseCase struct to activeMQInteractor to encapsulate the implementation details.
- Usecase: Fixed a potential resource leak in GetFullMessageBody by catching and bubbling up temporary queue deletion errors using named return variables.
- Build: Resolved ST1005 golangci-lint warning by removing trailing punctuation from error strings.