fix: make repeater stats battery display board-aware

docs/web-panel.md

@@ -224,6 +224,8 @@ For boards that expose extra telemetry, the optional `Environment` summary card
 
 Metrics with no current value are hidden rather than showing placeholder rows, so the cards vary by board and by current sensor state.
 
+The `Core` battery meter prefers a board-reported battery percentage when the target exposes one. Otherwise it scales the displayed percentage from the board's configured battery voltage range rather than assuming a fixed single-cell `3000-4200 mV` pack.
+
 The trend graphs load sequentially rather than as one large payload:
 
 1. summary/status