Fix hashrate sensor unit: H/s → GH/s

[Copilot]

The AxeOS API reports hashrate values in GH/s, but all hashrate sensors were labeled with H/s, causing incorrect unit display in Home Assistant.

Changes

• sensor.py: Updated unit from "H/s" to "GH/s" for all six hashrate sensor definitions:
  • hashRate, hashRate_1m, hashRate_10m, hashRate_1h, hashRate_1d, expectedHashrate

# Before
"hashRate": ("Current Hashrate", "H/s", ["hashRate"], ...),

# After
"hashRate": ("Current Hashrate", "GH/s", ["hashRate"], ...),

• README.md: Updated unit references in the sensor list and the automation example message template.

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