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/App.svelte

@@ -19,6 +19,7 @@
   import MutawalliDashboard from './lib/MutawalliDashboard.svelte';
   import FamilyTree from './lib/FamilyTree.svelte';
   import InsurancePolicies from './lib/InsurancePolicies.svelte';
+  import ZakatCalculator from './lib/ZakatCalculator.svelte';
 
   let currentLang = $state('en');
   lang.subscribe(v => currentLang = v);
@@ -42,8 +43,8 @@
     }
   });
 
-  const tabs = ['Coverage', 'Faraid', 'Assets', 'Insurance', 'Wassiyah', 'Hibah', 'Family Waqf', 'Nominate', 'Trigger', 'Mutawalli', 'Tree', 'Claims (H2)', 'Family', 'Settings'];
-  const icons = ['🎯', '📊', '📁', '🛡️', '📜', '🎁', '⛲', '📇', '⚡', '🕋', '🌳', '🔗', '👥', '⚙️'];
+  const tabs = ['Coverage', 'Faraid', 'Assets', 'Insurance', 'Zakat', 'Wassiyah', 'Hibah', 'Family Waqf', 'Nominate', 'Trigger', 'Mutawalli', 'Tree', 'Claims (H2)', 'Family', 'Settings'];
+  const icons = ['🎯', '📊', '📁', '🛡️', '🌙', '📜', '🎁', '⛲', '📇', '⚡', '🕋', '🌳', '🔗', '👥', '⚙️'];
   let activeTab = $state(0);
 
   function handleKeydown(e) {
@@ -101,16 +102,17 @@
     {:else if activeTab === 1}<FaraidCalculator />
     {:else if activeTab === 2}<AssetRegistry />
     {:else if activeTab === 3}<InsurancePolicies />
-    {:else if activeTab === 4}<WassiyahGenerator />
-    {:else if activeTab === 5}<HibahTracker />
-    {:else if activeTab === 6}<FamilyWaqfDesignator />
-    {:else if activeTab === 7}<NominationRegistry />
-    {:else if activeTab === 8}<DeathTrigger />
-    {:else if activeTab === 9}<MutawalliDashboard />
-    {:else if activeTab === 10}<FamilyTree />
-    {:else if activeTab === 11}<DigitalClaims />
-    {:else if activeTab === 12}<FamilyManagement />
-    {:else if activeTab === 13}
+    {:else if activeTab === 4}<ZakatCalculator />
+    {:else if activeTab === 5}<WassiyahGenerator />
+    {:else if activeTab === 6}<HibahTracker />
+    {:else if activeTab === 7}<FamilyWaqfDesignator />
+    {:else if activeTab === 8}<NominationRegistry />
+    {:else if activeTab === 9}<DeathTrigger />
+    {:else if activeTab === 10}<MutawalliDashboard />
+    {:else if activeTab === 11}<FamilyTree />
+    {:else if activeTab === 12}<DigitalClaims />
+    {:else if activeTab === 13}<FamilyManagement />
+    {:else if activeTab === 14}
       <div class="module">
         <div class="module-header">
           <h2>Settings</h2>