feat(deposits): pre-fill 30 slots and add back-page overflow for 31-60 checks

- Deposit panel now pre-fills all 30 check slots on open (new and existing)
- Remove button maintains 30-slot minimum by appending a blank row
- Add Row button hidden at <30, visible at 30-59, disabled at 60
- Deposit slip PDF splits items: first 30 on front, up to 30 more on back
- Front page gains a FROM REVERSE row carrying back-page subtotal when needed
- Back page renders with same column positions/width as front, titled
  'ADDITIONAL CHECK LISTING', numbered rows 31-60, and
  'Forward to other side / TOTAL $' footer

Closes #10, closes #11

public/js/app.js

@@ -1442,14 +1442,18 @@ async function openDepositPanel(id = null) {
       document.getElementById('dep-coin').value     = dep.coin      || '';
       document.getElementById('dep-cashback').value = dep.cash_back || '';
       depState.items = (dep.items || []).map(it => ({ ...it }));
+      while (depState.items.length < 30) depState.items.push(newDepItem());
     } catch (err) {
       alert('Error loading deposit: ' + err.message);
       return;
     }
   } else {
-    depState.items = [newDepItem()];
+    depState.items = [];
   }
 
+  // Always start with at least 30 slots (one full deposit slip page)
+  while (depState.items.length < 30) depState.items.push(newDepItem());
+
   renderDepItems();
   recalcDepTotals();
 
@@ -1475,6 +1479,13 @@ function newDepItem() {
 }
 
 function renderDepItems() {
+  const addBtn = document.getElementById('btn-add-dep-item');
+  if (addBtn) {
+    const count = depState.items.length;
+    addBtn.hidden    = count < 30;   // only show when already at 30 (back-page territory)
+    addBtn.disabled  = count >= 60;
+    addBtn.textContent = count >= 60 ? '+ Add Row (max 60)' : '+ Add Row (back page)';
+  }
   const tbody = document.getElementById('dep-items-tbody');
   tbody.innerHTML = depState.items.map((item, i) => `
     <tr data-idx="${i}">
@@ -1497,6 +1508,8 @@ function renderDepItems() {
   tbody.querySelectorAll('.dep-item-remove').forEach(btn => {
     btn.addEventListener('click', () => {
       depState.items.splice(parseInt(btn.dataset.idx, 10), 1);
+      // Maintain 30-slot minimum (one full slip page)
+      while (depState.items.length < 30) depState.items.push(newDepItem());
       renderDepItems();
       recalcDepTotals();
     });
@@ -1900,6 +1913,7 @@ async function init() {
   document.getElementById('dep-panel-overlay').addEventListener('click', closeDepositPanel);
   document.getElementById('btn-save-deposit').addEventListener('click', saveDeposit);
   document.getElementById('btn-add-dep-item').addEventListener('click', () => {
+    if (depState.items.length >= 60) return;
     depState.items.push(newDepItem());
     renderDepItems();
   });