Add remaining-weight input mode to inventory scan/usage page

Users can now toggle between 'Amount Used' and 'Remaining Weight' on the
QR scan page. In remaining-weight mode, usage is calculated as
(current stock - remaining) before submit — no controller changes needed.
Includes live hint showing calculated usage and new balance as they type,
with validation preventing negative usage or remaining > current stock.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Inventory/Scan.cshtml

@@ -168,6 +168,23 @@
         }
         .reason-pill.selected { border-color: var(--purple); background: #f3effe; color: var(--purple); font-weight: 600; }
 
+        /* ── Input mode toggle ───────────────────────── */
+        .mode-toggle { display: flex; border: 1.5px solid var(--border); border-radius: 8px; overflow: hidden; margin-bottom: 18px; }
+        .mode-btn {
+            flex: 1;
+            padding: 10px 8px;
+            background: #fff;
+            border: none;
+            font-size: 13px;
+            font-weight: 600;
+            color: var(--muted);
+            cursor: pointer;
+            text-align: center;
+            transition: background .15s, color .15s;
+        }
+        .mode-btn.active { background: var(--purple); color: #fff; }
+        .mode-btn:first-child { border-right: 1.5px solid var(--border); }
+
         /* ── Submit / Cancel ─────────────────────────── */
         .btn-submit {
             width: 100%;
@@ -309,12 +326,28 @@
 
         <div class="form-card">
             <h2>2. Enter Quantity</h2>
-            <div class="field">
+
+            <div class="mode-toggle">
+                <button type="button" class="mode-btn active" id="modeUsed" onclick="setMode('used')">Amount Used</button>
+                <button type="button" class="mode-btn" id="modeRemaining" onclick="setMode('remaining')">Remaining Weight</button>
+            </div>
+
+            <!-- amount-used mode -->
+            <div id="usedField" class="field">
                 <label for="quantityInput">Amount Used (@item.UnitOfMeasure) <span class="req">*</span></label>
                 <input type="number" id="quantityInput" name="quantity"
-                       min="0" step="any" required placeholder="0" inputmode="decimal" />
+                       min="0" step="any" placeholder="0" inputmode="decimal"
+                       oninvalid="this.setCustomValidity('')" />
                 <div class="hint" id="balanceHint"></div>
             </div>
+
+            <!-- remaining-weight mode -->
+            <div id="remainingField" class="field" style="display:none">
+                <label for="remainingInput">Weight Remaining (@item.UnitOfMeasure) <span class="req">*</span></label>
+                <input type="number" id="remainingInput" min="0" step="any"
+                       placeholder="0" inputmode="decimal" />
+                <div class="hint" id="remainingHint"></div>
+            </div>
         </div>
 
         <div class="form-card">
@@ -346,6 +379,21 @@
     <script>
     var currentQty = @item.QuantityOnHand;
     var uom = '@item.UnitOfMeasure';
+    var inputMode = 'used'; // 'used' | 'remaining'
+
+    // ── Input mode toggle ────────────────────────────
+    function setMode(mode) {
+        inputMode = mode;
+        document.getElementById('modeUsed').classList.toggle('active', mode === 'used');
+        document.getElementById('modeRemaining').classList.toggle('active', mode === 'remaining');
+        document.getElementById('usedField').style.display = mode === 'used' ? '' : 'none';
+        document.getElementById('remainingField').style.display = mode === 'remaining' ? '' : 'none';
+        document.getElementById('balanceHint').textContent = '';
+        document.getElementById('remainingHint').textContent = '';
+        // clear both inputs when switching
+        document.getElementById('quantityInput').value = '';
+        document.getElementById('remainingInput').value = '';
+    }
 
     // ── Job selection ────────────────────────────────
     function showTab(tab) {
@@ -384,7 +432,7 @@
         document.getElementById('transactionTypeInput').value = el.dataset.val;
     }
 
-    // ── Balance hint ─────────────────────────────────
+    // ── Balance hint (amount-used mode) ─────────────
     document.getElementById('quantityInput').addEventListener('input', function() {
         var qty = parseFloat(this.value) || 0;
         if (!this.value) { document.getElementById('balanceHint').textContent = ''; return; }
@@ -394,6 +442,24 @@
             'New balance: <strong style="color:' + col + '">' + newBal.toFixed(2) + ' ' + uom + '</strong>';
     });
 
+    // ── Remaining-weight hint ────────────────────────
+    document.getElementById('remainingInput').addEventListener('input', function() {
+        var hint = document.getElementById('remainingHint');
+        if (!this.value) { hint.textContent = ''; return; }
+        var remaining = parseFloat(this.value);
+        if (isNaN(remaining) || remaining < 0) { hint.innerHTML = '<span style="color:var(--danger)">Enter a valid weight.</span>'; return; }
+        if (remaining > currentQty) {
+            hint.innerHTML = '<span style="color:var(--danger)">Remaining cannot exceed current stock (' + currentQty.toFixed(2) + ' ' + uom + ').</span>';
+            return;
+        }
+        var used = currentQty - remaining;
+        if (used <= 0) {
+            hint.innerHTML = '<span style="color:var(--danger)">No usage to log &mdash; remaining equals current stock.</span>';
+            return;
+        }
+        hint.innerHTML = 'Will log <strong>' + used.toFixed(2) + ' ' + uom + '</strong> as used &mdash; new balance: <strong style="color:' + (remaining === 0 ? '#343a40' : 'var(--success)') + '">' + remaining.toFixed(2) + ' ' + uom + '</strong>';
+    });
+
     // ── Preselect job if coming from success page ────
     @if (preselectedJobId.HasValue)
     {
@@ -406,8 +472,37 @@
         </text>
     }
 
-    // ── Submit spinner ───────────────────────────────
-    document.getElementById('usageForm').addEventListener('submit', function() {
+    // ── Submit: resolve quantity from whichever mode is active ──
+    document.getElementById('usageForm').addEventListener('submit', function(e) {
+        if (inputMode === 'remaining') {
+            var remaining = parseFloat(document.getElementById('remainingInput').value);
+            if (isNaN(remaining) || remaining < 0 || remaining > currentQty) {
+                e.preventDefault();
+                document.getElementById('remainingHint').innerHTML =
+                    '<span style="color:var(--danger)">Please enter a valid remaining weight.</span>';
+                return;
+            }
+            var used = currentQty - remaining;
+            if (used <= 0) {
+                e.preventDefault();
+                document.getElementById('remainingHint').innerHTML =
+                    '<span style="color:var(--danger)">No usage to log &mdash; remaining equals current stock.</span>';
+                return;
+            }
+            document.getElementById('quantityInput').value = used.toFixed(4);
+        }
+
+        // validate amount-used mode
+        if (inputMode === 'used') {
+            var qty = parseFloat(document.getElementById('quantityInput').value);
+            if (isNaN(qty) || qty <= 0) {
+                e.preventDefault();
+                document.getElementById('balanceHint').innerHTML =
+                    '<span style="color:var(--danger)">Please enter a quantity greater than zero.</span>';
+                return;
+            }
+        }
+
         var btn = document.getElementById('submitBtn');
         btn.disabled = true;
         btn.textContent = 'Saving…';