Fix simple-at-maturity payout bug and minor issues

- Fix: simple-at-maturity final payout now includes total interest
  (was only paying one year's interest due to dead ternary)
- Fix: add type='button' to prevent form submission in WordPress
- Fix: add radix 10 to parseInt
- Fix: remove misleading tilde on exact average income calculation

calc.html

@@ -77,7 +77,7 @@
   <!-- Triggers the run() function which validates, computes, and     -->
   <!-- renders the results.                                            -->
   <!-- =============================================================== -->
-  <button id="scd-btn"
+  <button type="button" id="scd-btn"
     style="display:block;width:100%;padding:18px;background:#e66d00;color:#fff;font-size:20px;font-weight:700;border:none;border-radius:10px;cursor:pointer;margin-bottom:8px;letter-spacing:0.3px;">
     Calculate
   </button>
@@ -210,7 +210,7 @@
     /* --- Read raw inputs --- */
     var a=parseFloat(document.getElementById('scd-amt').value),      /* Investment amount (£) */
         r=parseFloat(document.getElementById('scd-rate').value),     /* Annual interest rate (%) */
-        t=parseInt(document.getElementById('scd-term').value),       /* Bond term (years) */
+        t=parseInt(document.getElementById('scd-term').value,10),    /* Bond term (years) */
         mEl=document.querySelector('input[name="scd-m"]:checked'),  /* Selected payout method */
         m=mEl?mEl.value:'cm',                                       /* Default to compound-at-maturity */
         e=document.getElementById('scd-err'),                        /* Error container */
@@ -245,7 +245,7 @@
       if(ann){
         p=i;                                       /* yearly payout = this year's interest */
       }else if(y===t){                              /* final year for at-maturity modes */
-        p=cmp?bal+i:bal+i;                          /* pay out current balance + final interest */
+        p=cmp?bal+i:a+totI;                         /* cm: full accumulated balance; sm: principal + total simple interest */
       }
       totP+=p;
 
@@ -281,7 +281,7 @@
     /* --- Populate summary cards --- */
     document.getElementById('scd-r-total').textContent=g(total);
     document.getElementById('scd-r-interest').textContent=g(netI);
-    document.getElementById('scd-r-annual').textContent=(ann?'':'~')+g(yrInc)+(ann?'/yr':'/yr avg');
+    document.getElementById('scd-r-annual').textContent=g(yrInc)+(ann?'/yr':'/yr avg');
     document.getElementById('scd-r-eff').textContent=eff.toFixed(2)+'%';
 
     /* --- Update security tier display --- */