[Ryankhlifi]

Summary

cpuTempC now reads sensors through gopsutil and pins one sensor for the
lifetime of the process, so successive readings always come from the same
source.

Fixes #6

Why

gopsutil doesn't guarantee the order of the sensors it returns, so picking
temps[0] (or the first match) can return a different sensor from one call to
the next. Package and per-core temperatures can differ by several degrees, so
the displayed value could jump without any real change in temperature.

Changes

• Sensors are matched against cpuSensorPrefixes in preference order, so
  package-level sensors win over per-core ones. The first successful call pins
  that sensor's full SensorKey.
• Later calls read only the pinned sensor. If it is missing or reads 0, the
  function returns (0, false) rather than falling back to another sensor.
• The pinned key is guarded by a mutex, so concurrent callers are safe.
• Replaces the direct /sys/class/thermal read with
  github.com/shirou/gopsutil/v4/sensors (pure Go, no cgo). go.mod and
  go.sum are updated.
• Doc comments rewritten to describe the above.

Behavior to be aware of

• (0, false) is returned on any error from SensorsTemperatures (including
  warnings about individual unreadable sensors) and when no sensor matches.
  This is the normal result on VMs, containers, WSL, and Windows.
• A pinned sensor that disappears for good (for example, a driver reload that
  renames it) means no temperature until restart. This is deliberately strict;
  I can add a reset after N consecutive misses if you'd prefer.
• On Windows, gopsutil exposes only an ACPI thermal zone, and only when
  elevated. It isn't guaranteed to be the CPU, so I left it out of
  cpuSensorPrefixes. The function reports a real CPU measurement or says it
  can't.

Testing

• Linux (fake sysfs): no hardware with sensors was available, so I used a
  fake sysfs tree via HOST_SYS containing coretemp_package_id_0 and
  coretemp_core_0. The package sensor was selected and its reading
  displayed.
• WSL: no sensors exposed, so (0, false).
• Windows: without elevation, gopsutil returns "Access denied". Elevated,
  it exposes one ACPI thermal zone (ACPI\ThermalZone\TZ00_0) that doesn't
  reliably map to the CPU, so cpuTempC returns (0, false) and the UI shows
  n/a.
• go vet ./... and go test ./... pass on Linux/WSL. On Windows,
  TestKeyFilesAreNotWorldReadable and TestStateFileIsPrivateAndValid fail
  because Go reports 0666/0777 there regardless of real permissions. They
  are unrelated to this change, and I left them alone to keep the PR focused.
• No new unit tests are included. I'm happy to add tests for the sensor
  pinning if you'd like them.
• A check on physical Linux hardware would be welcome.