Add field selectors for charge, load, and state metrics

Field Mapping options (new category at top of editor):
- Charge field: pick which numeric field drives the battery level
  (falls back to first number field if unset)
- Enable load metric: toggle + field picker for a load/power draw
  percentage, displayed as a themed semi-circular gauge arc beside
  the battery (green < 50%, amber 50-80%, red > 80%)
- Enable state field: toggle + field picker + user-defined values
  for charging/discharging so the panel uses an explicit data field
  rather than computing state from the rate of change

Visual integration:
- Load gauge renders in a card with the battery, scales responsively,
  and collapses to a compact text line in small panels
- State field drives the status badge, gradient direction, and
  charging animation (overrides computed rate when enabled)
- All new options use showIf for conditional visibility in the editor

Also fixed deprecation warnings by replacing .toArray() with
Array.from() for Grafana 12+ compatibility.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/types.ts

@@ -1,6 +1,15 @@
 export type BatteryOrientation = 'vertical' | 'horizontal';
 
 export interface ChargeVizOptions {
+  // Field selectors
+  chargeField: string;
+  enableLoad: boolean;
+  loadField: string;
+  enableStateField: boolean;
+  stateField: string;
+  stateChargingValue: string;
+  stateDischargingValue: string;
+
   // Colours
   lowColour: string;
   midColour: string;