feat: flash dev auto-selects next available port

[KAJdev]

When the default port (8888) or the user-specified --port is already in use, flash dev now automatically tries the next port, counting up to 20 attempts before failing.

Previously, flash dev would crash with [Errno 48] Address already in use.

Changes:

• Add _find_available_port(host, start_port) that probes ports via socket.bind() before launching uvicorn
• If a different port is selected, prints a yellow notice: Port 8888 is in use, using 8889 instead.
• Fails with a clear error after 20 consecutive ports are exhausted
• Existing --port / -p flag behavior is unchanged (it sets the starting port)

Closes AE-2220