Live spectrogram, button icons, check-for-updates button

[BigWebstas]

What

• Spectrogram: scrolling spectrogram on the main screen while recording (frozen on pause, hidden when idle). It uses its own AudioRecord plus an in-house FFT, runs only between onStart and onStop, and is Android 10+ only because it needs a second mic capture next to the recorder's.
• Icons: Material icons on the folder, install-on-Pebble and check-for-updates buttons (same watch / system_update icons as PebbleSleepTracker).
• Update button: "Check for updates" in the github flavor. It reuses the launch-time flow and always reports the result.
• Label: the watchapp button now reads "Install on Pebble".

Testing

• Both flavors build.
• FFT and level scaling checked with test tones (1, 3 and 6 kHz land on the right rows).
• Release build installed on a Pixel 9 Pro XL: buttons and icons verified by screenshot, and recordings made with the screen open sound fine.

🤖 Generated with Claude Code