Fix landing-page growth %: read online_nodes (not total_nodes) from polo daily series

Per-day entries in /api/stats carry `online_nodes`; the calculation was
reading `total_nodes` from each row, hitting NaN, and falling through to
the build-time placeholder (+100%). Switched both build-time and runtime
paths to read online_nodes with a fallback to top-level total_nodes for
the latest day, so the figure now reflects real 7-day growth (~+56% at
the moment of this change). Lowered the build-time fallback from +100%
to +50% as a more defensible static value.

Author: teovl