Fix power button page turn on long-press sleep

[niklasarnitz]

Summary

• What is the goal of this PR?
  Fix a small UX issue: when short power-button press is configured as page turn, putting the device to sleep via long press should not also turn a page.
• What changes are included?
  ReaderState now tracks how long the power button is held when short-press page turn is enabled.
  Page turn is deferred until release and only triggered for true short presses; long presses are treated as sleep action only.

Additional Context

• Add any other information that might be helpful for the reviewer (e.g., performance implications, potential risks, specific areas to focus on).