Fix GDScript ternary and shadowed-identifier warnings in dashboard scripts

Godot's static checker can't unify a float branch with a null branch in
a ternary, so point_value() now uses explicit if/return; renaming the
local sign variable to sign_prefix avoids shadowing the builtin sign().

scripts/ui/dashboard_chart.gd

@@ -99,9 +99,13 @@ func _draw() -> void:
 
 static func point_value(point: Dictionary, series_mode: String) -> Variant:
 	if series_mode == "accuracy":
-		return float(point.get("accuracy", 0)) if int(point.get("total", 0)) > 0 else null
+		if int(point.get("total", 0)) > 0:
+			return float(point.get("accuracy", 0))
+		return null
 	var value: Variant = point.get("questionBankRating", null)
-	return null if value == null else float(value)
+	if value == null:
+		return null
+	return float(value)
 
 
 static func axis_bounds(series_points: Array, series_mode: String) -> Vector2: