fix(power): use BQ25896 ADC for both battery voltage and presence

[DatanoiseTV]

getBattVoltage on the BQ27220+BQ25896 combo returned the fuel-gauge
reading while isBatteryConnect derived presence from the BQ25896
charger ADC. The two could disagree on a freshly-attached cell before
the fuel gauge had learned: hasBattery would say false but voltage
4.0V, and the upstream PowerFSM would treat the unit as externally
powered. Read voltage from the same source as presence so both views
move together.

Split out from #10425 — single-concern PR.

Build verification

pio run -e t-deck-tft succeeds, no new warnings.

Attestations

• I have tested that my proposed changes behave as described — review/static-analysis only, not on-air.
• On-hardware testing requested from community: build-verified t-deck-tft only.

[mverch67]

@DatanoiseTV you wrote:

The T-Deck does not have a BQ27220/BQ25896 charger/gauge, it uses GPIO ADC battery pin 4.

Please use T-Lora Pager or T-Deck Pro for tests with BQ27220/BQ25896.

Also, the BQ27220 has a more accurate voltage reporting as it implements the calibrated charging curve and automatically learns the charge capacity from previous full battery cycles.

[NomDeTom]

Be very careful - the bq25896 is currently linked to another bq module for "fuel gauging" and also shares an i2c address with some other common sensors.

[mverch67]

yeah, T-Deck I2C keyboard and BQ27220 also have the same I2C address 0x55. Maybe that's where the confusion came from.