feat: add Zakat calculator (per-member, Nisab + 2.5% rate)

Closes the Zakat gap identified in the competitive benchmark against
Rafiq. Per-member module (nf_zakat_records, same author-owns/family-reads
RLS pattern as Insurance/Wassiyah/Waqf) computing 2.5% due on zakatable
wealth (cash, gold, silver, business assets, investments) once above a
user-entered Nisab threshold, minus deductible short-term liabilities.
Manual entry only, matching the app's existing philosophy — no live gold
price feed. Covered by e2e-zakat.cjs (7/7).

src/lib/calc/zakat.js

@@ -0,0 +1,24 @@
+// Shared Zakat calculation core — Nisab check + 2.5% (1/40) on qualifying
+// wealth held above threshold. Nisab is entered as a value, not a live gold
+// price feed — matches the app's "manual entry only" philosophy already
+// established for Asset Registry / Faraid.
+const ZAKAT_RATE = 0.025;
+
+export function zakatableWealth(fields) {
+  const gross = (Number(fields.cash) || 0) + (Number(fields.gold) || 0) + (Number(fields.silver) || 0)
+    + (Number(fields.businessAssets) || 0) + (Number(fields.investments) || 0) + (Number(fields.otherZakatable) || 0);
+  const deductible = Number(fields.deductibleLiabilities) || 0;
+  return Math.max(0, gross - deductible);
+}
+
+export function zakatDue(fields) {
+  const wealth = zakatableWealth(fields);
+  const nisab = Number(fields.nisabThreshold) || 0;
+  if (nisab <= 0 || wealth < nisab) return 0;
+  return wealth * ZAKAT_RATE;
+}
+
+export function meetsNisab(fields) {
+  const nisab = Number(fields.nisabThreshold) || 0;
+  return nisab > 0 && zakatableWealth(fields) >= nisab;
+}