Update landing page and ground link features

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

src/core/rebar/column.ts

@@ -50,7 +50,7 @@ export interface ColumnInput {
 export function columnInputFor(model: PierModel, base: ColumnInput = COLUMN_P1): ColumnInput {
   const col = model.columns[0];
   const seg = model.segments.find((g) => col.s >= g.s0 - 1e-9 && col.s <= g.s1 + 1e-9) ?? model.segments[0];
-  const Hc = Math.round((model.levelBottom - model.footings[0].top) * 1000 * 10) / 10;
+  const Hc = Math.round((model.levelBottom - (model.footings[0]?.top ?? col.bottom)) * 1000 * 10) / 10;
   const hcop = Math.round((seg.top - model.levelBottom) * 1000 * 10) / 10;
   // C3 (상면 3단) = the lowest coping top −127 (C1 cover) −2·s_lay (상면 주철근 단 간격 input); P1 was 43.170 at 57.2.
   // P1 top = C3 − 100 (계산 2026-10-23), so e_P1 = step top − (C3 − 0.1) = 485 on P1.