Proper notification icons (thermometer for temp alert, state-aware status icon)

[almothafar]

Summary

A few notifications added during the recent work reuse the existing "battery charging" status glyphs (ic_stat_device_battery_charging_20/50/full) as placeholders, because the project had no better icons. They work fine, but the icon isn't semantically right. This is cosmetic polish, not a bug/regression — just a deliberate placeholder noted in the original PRs (#5, #18).

Where

• Temperature alert (NotificationService.sendTemperatureNotification, ~line 199): shows ic_stat_device_battery_charging_20 — a charging icon for a heat alert. Should be a thermometer / hot-battery icon.
• Persistent status notification (NotificationService.ongoingIconRes, ~line 735, used at ~line 258): picks a charging glyph by level, so it can show a charging bolt even while discharging. Should reflect actual state (a plain battery-level icon, with a bolt only when charging).

(For reference, the critical/warning/full alert icons in NotificationConfig also use charging-themed glyphs, but that's the app's pre-existing design — in scope only if we do a full set.)

Proposed

• Add a small, monochrome status-bar icon set (Android status icons must be white-on-transparent):
  • a thermometer / hot icon for the temperature alert,
  • a battery-level icon (optionally a few level buckets) for the ongoing status notification, with a charging variant.
• Optionally unify the critical/warning/full alert icons into the same set.

Acceptance criteria

• The temperature alert shows a heat/thermometer icon, not a charging glyph.
• The persistent notification's icon reflects charging vs. discharging correctly.

Follow-up polish from #5 (persistent notification) and #18 (temperature alert).