feat: Improve thermal display and build system

- Replace emoji thermal indicators with modern progress bar UI
- Switch temperature source to battery sensor for better accuracy
- Adjust temperature thresholds (30°C-45°C range)
- Add automatic Vulkan Validation Layer download for Android
- Downgrade Java/Kotlin target to 17 for wider compatibility

The thermal display now shows a visual progress bar with percentage
instead of emojis, making it easier to gauge system temperature at
a glance. Temperature reading now comes from the battery sensor
which is more reliable across devices.

Build system improvements include automated VVL binary downloads
and installation for Android builds when CITRON_DOWNLOAD_ANDROID_VVL
is enabled. Java target downgraded to 17 to ensure compatibility
with current Android toolchain.

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