Changed from fixed padding (0.5ct below min, 1ct above max) to
proportional padding based on data range (8% below, 15% above).
This ensures consistent visual "airiness" across all price ranges,
whether prices are at 30ct or 150ct. Both subunit (ct/øre) and
base currency (€/kr) now use the same proportional logic.
Previous fixed padding looked too tight on charts with large price
ranges (e.g., 0.6€-1.5€) compared to charts with small ranges
(e.g., 28-35ct).
Impact: Chart metadata sensor provides better-scaled yaxis_min/yaxis_max
values for all chart cards, making price visualizations more readable
with appropriate whitespace around data regardless of price range.