[windows] host stats: CPU and load averages are never read; the UI shows them as measured zeros #139

On Windows, host.Reader.Read() (internal/host/host_windows.go) fills RAM and swap from GlobalMemoryStatusEx and reports the real core count, but CPU and the load averages have no reading and "stay 0" (per the comment in the file). The sample still marks the reader supported, so the harbor card's machine meter renders CPU 0% and LOAD 0 0 0 as live readings.

Repro (main @ 050866f, Windows 11, one RTX 5070 Ti decoding a 27B model): last /v1/history sample while the GPU is at 91% util:

"cpu_pct": 0,
"ram_used": 30759657472

The card shows CPU 0% and zero load next to the real RAM figure — it reads as a dead gauge rather than "not measured on this platform".

Two parts:

1. cpu_pct is cheap to read on Windows: two GetSystemTimes reads between polls give idle vs total time. Load averages have no Windows equivalent; the UI should show n/a for those.
2. The UI cannot distinguish "unsupported" from "zero". The card should render n/a for fields the platform does not read (the same page already has an n/a helper for this).