FW: Show red precharge indication when startup battery check fails with charger connected

[TobiasRoeddiger]

Summary

When the device boots with charger power connected but the battery check fails because the fuel gauge is still in system-down / precharge range, the firmware should immediately show the red pulsing precharge indication.

Currently the startup path either shows POWER_CONNECTED briefly or powers down before the normal battery state machinery can publish PRECHARGING, so users may see no red pulse even though the charger is still precharging the cell.

Current behavior

In PowerManager::begin():

• check_battery() may return false when the BQ27220 reports SYSDWN.
• If charger power is connected, the code continues into the charging branch.
• That branch initializes oe_state.charging_state = POWER_CONNECTED, which maps to orange.
• PRECHARGING is only set later from fuel_gauge_work_handler() when charger state is charging and bat.SYSDWN is true.

Relevant code:

• src/Battery/PowerManager.cpp: startup battery check and charging branch around check_battery(), POWER_CONNECTED, and state_indicator.init().
• src/Battery/PowerManager.cpp: fuel_gauge_work_handler() maps charging + SYSDWN to PRECHARGING.
• src/utils/StateIndicator.cpp: PRECHARGING maps to led_controller.pulse(LED_RED, ...).

Expected behavior

If battery check fails but charger power is present, the device should enter a charging-only/preboot recovery mode and immediately set the indicator to PRECHARGING when the failing reason is compatible with precharge/system-down recovery.

This should make the LED behavior match the README entry:

Red pulsing: Pre-charge phase or system-down voltage not yet cleared

Proposed implementation

1. Change the battery check from a plain boolean to a richer result/reason, for example:

   • OK
   • SYS_DOWN
   • VOLTAGE_TOO_LOW
   • TEMP_OUT_OF_RANGE
   • READ_ERROR

2. In PowerManager::begin():

   • If battery check fails and charger is not connected: power down as today.
   • If battery check fails and charger is connected:
     • initialize the state indicator immediately;
     • set oe_state.charging_state = PRECHARGING for SYS_DOWN / precharge-compatible failures;
     • set FAULT or another explicit state for non-recoverable failures such as unsafe temperature;
     • schedule the normal charge controller work and remain in the charger-present wait loop.

3. Keep fuel_gauge_work_handler() as the ongoing source of truth after startup. It can continue to update PRECHARGING, CHARGING, FAULT, etc. once periodic charger/fuel-gauge reads are available.

4. Improve I2C read error handling in the battery drivers so failed fuel-gauge reads do not silently become zero-like values that can be misclassified as real voltage, temperature, or flag data.

Why this matters

When a deeply discharged device is plugged in, it may be doing the correct hardware recovery/precharge behavior but provide no clear user feedback. Showing red pulse immediately in this mode makes the device state understandable and aligns runtime behavior with the documented battery LED states.