fix(chart): keep zero-year chara and ephemeris dates apart

A zero-year Chara period is shown as 0 years instead of failing the whole chart. Switching ephemeris dates drops the previous day immediately and can be retried. Missing birth data links to chart settings.

frontend/src/lib/chart-view-mapper.ts

@@ -82,11 +82,19 @@ function isoDay(value: string): string {
 function addWeightedSpan(startIso: string, endIso: string, index: number, weights: readonly number[]): { start: string; end: string } {
   const start = Date.parse(`${startIso}T00:00:00Z`);
   const end = Date.parse(`${endIso}T00:00:00Z`);
-  const total = weights.reduce((sum, weight) => sum + weight, 0);
-  const before = weights.slice(0, index).reduce((sum, weight) => sum + weight, 0);
+  const total = weights.reduce((sum, weight) => sum + (Number.isFinite(weight) ? weight : 0), 0);
+  // A zero-year Chara mahadasha gives every antardasha weight 0. `??` does not
+  // replace 0, so total is 0 and `before / total` is NaN. Never call
+  // `Date#toISOString` on that — it throws RangeError and fails the whole chart.
+  if (!Number.isFinite(start) || !Number.isFinite(end) || !(total > 0)) {
+    return { start: startIso, end: startIso };
+  }
+  const before = weights.slice(0, index).reduce((sum, weight) => sum + (Number.isFinite(weight) ? weight : 0), 0);
+  const weight = Number.isFinite(weights[index]) ? weights[index]! : 0;
   const span = end - start;
   const from = start + span * (before / total);
-  const to = index === weights.length - 1 ? end : start + span * ((before + weights[index]!) / total);
+  const to = index === weights.length - 1 ? end : start + span * ((before + weight) / total);
+  if (!Number.isFinite(from) || !Number.isFinite(to)) return { start: startIso, end: startIso };
   return { start: new Date(from).toISOString().slice(0, 10), end: new Date(to).toISOString().slice(0, 10) };
 }
 
@@ -390,12 +398,16 @@ function charaTrack(charaPacket: Record<string, unknown> | null, birthDate: stri
     if (!row) return [];
     const sign = text(row.sign);
     const lord = text(row.lord) ?? sign ?? "—";
-    const duration = finite(row.duration_years) ?? 1;
+    const rawDuration = finite(row.duration_years);
+    const duration = rawDuration === null ? 1 : rawDuration;
+    const zeroYear = duration === 0;
     const start = cursor;
     const end = addYears(start, duration);
     cursor = end;
     const rawAntar = Array.isArray(row.antardashas) ? row.antardashas : [];
-    const antardashas = rawAntar.length > 0
+    const antardashas = zeroYear
+      ? []
+      : rawAntar.length > 0
       ? rawAntar.flatMap((antar, index) => {
         const item = record(antar);
         if (!item) return [];
@@ -430,6 +442,7 @@ function charaTrack(charaPacket: Record<string, unknown> | null, birthDate: stri
       end,
       current: containsDay(start, end, asOf),
       derived: false,
+      ...(zeroYear ? { zeroYear: true as const } : {}),
       antardashas,
     }];
   });