feat(gui): add System temperature to Dashboard monitor

[ryann-g]

Summary

Adds a third System temperature reading (EC / board sensor) to the Dashboard's Temperature Monitor, alongside the existing CPU and GPU readings.

The linuwu_sense driver already registers an acer hwmon device exposing three temperature channels (src/linuwu_sense.c, acer_wmi_temp_channel_to_sensor_id[]):

Channel	Sensor	Currently shown by DAMX
temp1_input	ACER_WMID_SENSOR_CPU_TEMPERATURE	✅
temp2_input	ACER_WMID_SENSOR_GPU_TEMPERATURE	✅
temp3_input	ACER_WMID_SENSOR_EXTERNAL_TEMPERATURE_2	❌ (new)

This PR surfaces that third, independent EC-reported sensor in the GUI. It is not derived from CPU/GPU values and requires no driver or daemon changes.

Motivation

Users currently have no visibility into chassis/board temperature. The EC already reports it; the GUI simply never displayed it.

Changes

• Dashboard.axaml.cs
  • New FindSystemTempPath() — locates the acer hwmon device by scanning /sys/class/hwmon/*/name and reading temp3_input. Discovery is name-based rather than index-based, since hwmon numbering varies between boots/kernels.
  • Fallback chain: acer temp3_input → first non-CPU/GPU ACPI thermal zone → sensors acpitz regex (mirrors existing fallback style).
  • New GetSystemTemperature() following the same error-handling pattern as GetCpuTemperature().
  • Third LineSeries ("System Temperature", orange) on the Temperature Monitor chart + history buffer capped at MAX_HISTORY_POINTS.
  • MetricsData.SystemTemp wired through the existing refresh pipeline.
• Dashboard.axaml
  • Live CPU: x°C · GPU: x°C · System: x°C readout row inside the Temperature Monitor card.

Testing

Tested on Acer Nitro ANV15-41 (AMD Ryzen 5 7535HS + RTX 2050), Zorin OS 18.1, kernel 7.0.0-30-generic:

• ✅ dotnet build -c Release: 0 errors; no new functional warnings (the single added CS8618 matches the file's existing nullable-field pattern for _cpuTempHistory etc.)
• ✅ Discovery log at runtime:

  Found System Temperature at /sys/class/hwmon/hwmon4/temp3_input
  

• ✅ GUI readout matched sysfs live (temp3_input = 52000 m°C → displayed 52 °C), tracked across refresh cycles
• ✅ Chart renders three series; readout row renders above it
• ✅ Verified alongside the installed production DAMX instance running simultaneously — no interference (GUI-only change, read-only /sys access)

Screenshots

Sandbox build (left) vs installed release (right):

Sandbox build (with this PR)	Installed release (before)
Temperature Monitor shows CPU · GPU · System readout row and third chart series	Original two-sensor layout

Checklist

• Build passes (Release)
• Tested on real hardware (ANV15-41)
• No changes to driver, daemon, or installer
• Follows existing code style (discovery caching in _systemInfoPaths, Console.WriteLine logging, RunCommand fallback)
• Tested on Intel-based Nitro/Predator models (would appreciate maintainer/maintainer-community verification)