feat(ui): add Fabric-branded progress spinner during command execution

[Copilot]

📥 Pull Request

✨ Description of new changes

Adds a Fabric-branded animated spinner on stderr while CLI commands execute, providing visual feedback for long-running operations.

Design

• Spinner context manager in fab_ui.py — background daemon thread animates braille characters (⠋⠙⠹⠸⠼⠴⠦⠧⠇⠏) on stderr
• Fabric-branded colors — braille frames rendered in Fabric teal (#49C5B1), message text in grey, matching the CLI's existing print_fabric() and get_common_style() palette
• TTY-aware — no-op when stderr is piped (non-interactive)
• 300ms startup delay — fast commands (config, pwd) never flash a spinner
• Auto-clear before output — stop_active_spinner() called from all output functions (print_output_format(), print_output_error(), print_progress(), print_done(), print_warning(), and print_info()) so the spinner line is always cleaned up before any console output renders, including during LRO polling

Integration points

• main.py:_execute_command() — wraps func(args) for command-line mode
• fab_interactive.py:handle_command() — wraps func(args) for interactive REPL mode

# Both paths now use:
with fab_ui.Spinner():
    return subparser_args.func(subparser_args)

LRO overlap prevention

The spinner is guaranteed not to overlap with Long Running Operation (LRO) polling output. Every output function used by the LRO flow — print_progress(), print_done(), print_warning(), print_info(), print_output_format(), and print_output_error() — calls stop_active_spinner() as its first action, clearing the spinner line from stderr before rendering status text. The call is idempotent, so it's safe when the spinner is already stopped or was never started.

Tests

11 unit tests covering init, TTY/non-TTY behavior, start/stop lifecycle, idempotent stop, global state management, and fast-command animation skip.

No new dependencies.

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