feat: battery monitoring, external RTC persistence, SD used-space fix

- Add BQ27220 fuel gauge: reads SOC percentage every 5s
- Add BQ25896 PMIC: detects charging status (pre-charge/fast/done)
- Add MAX31329 external RTC: reads time on boot, syncs from NTP
- Battery UI: icon + percentage in header, cyan when charging
- Fix SD used bytes: fallback to recursive dir walk when usedBytes() returns 0
- Libraries: PMIC_BQ25896, kode_bq27220, kode_MAX31329

platformio.ini

@@ -65,3 +65,6 @@ lib_deps =
   https://github.com/bitbank2/bb_captouch.git
   https://github.com/xreef/SimpleFTPServer.git
   bblanchon/ArduinoJson @ ^7
+  https://github.com/sqmsmu/PMIC_BQ25896.git
+  https://github.com/kodediy/kode_bq27220.git
+  https://github.com/kodediy/kode_MAX31329.git