Enhancement: GPU detection should support Apple Silicon (Metal/unified memory)

[mrveiss]

Problem

PR #1989 (#1959) expanded GPU detection to NVIDIA, AMD, and Intel — but Apple Silicon Macs are not detected. Since Python runs everywhere and users may deploy AutoBot on macOS, Apple Silicon's unified memory GPU should be recognized.

What's missing

• No Metal framework detection
• No unified memory reporting (M1/M2/M3/M4 share system RAM with GPU)
• No Apple GPU name detection (e.g., "Apple M2 Pro 19-core GPU")

Detection approach

On macOS, use:
```python
import platform
if platform.system() == "Darwin":
# system_profiler SPDisplaysDataType -json
# Returns chipset, VRAM, Metal support
```

Or via `subprocess`:
```bash
system_profiler SPDisplaysDataType -json
```

Context

User confirmed macOS is a target platform since Python runs everywhere.

Impact

Severity: medium — Mac users get `gpu_available=False`, missing Metal acceleration.