Add backlight control functionality for E Ink display using the touch button

[viperet]

This PR adds the ability to control the EINK backlight using the touch button on BaseUI, similar to InkHUD.

• Short-press enables the backlight for 3 seconds.
• Long press toggles backlight on/off.

Video: https://github.com/user-attachments/assets/ebe36cd2-2a41-4fa6-92be-3e5a81493ce8

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • Other (please specify below)
  • LiliGo T-Echo

[rcarteraz]

@thebentern haha looks like someone else had the same idea as you.

[viperet]

@rcarteraz, the backlight control that @thebentern implemented can work not only for T-Echo Plus, but also for regular T-Echo. But my implementation is more reusable and mimics InkHUD's backlight control.