Suspend/resume power management (B13)

[Ashex]

Context

sKeets has no power management. The device stays fully awake while the app runs, draining the battery. The src/platform/power.cpp infrastructure probes battery state and suspend safety, but no actual suspend action is triggered. Power button press during bootstrap just exits the app.

KOReader's suspend/resume state machine was analyzed in docs/koreader-cross-reference.md as reference.

Sub-tasks

• Detect inactivity timeout (e.g. 5 minutes with no touch/key input) → blank screen
• On inactivity timeout or power button short-press: tear down Wi-Fi, turn off frontlight, write mem to /sys/power/state for suspend-to-RAM
• Handle wake from suspend: restore Wi-Fi, restore frontlight, resume event loop
• Gate suspend safety: skip suspend if MTK device is charging (known kernel crash per KOReader reference)
• Write 1 to /sys/power/state-extended before suspend, 0 after resume (per KOReader pattern)
• Add unexpected wakeup guard: if wakeup was not user-initiated, re-suspend (with a max retry count)
• Handle sleep cover events (SleepCoverClosed → suspend, SleepCoverOpened → resume) if hardware supports it

References

• KOReader suspend sequence: device/kobo/device.lua (~L1330)
• Existing infra: src/platform/power.h/cpp (probes battery, charging, standby capability)
• MTK crash workaround: KOReader explicitly skips suspend/standby if isMTK() and isCharging()