feat(manual-batch): require plate number at manual mode start

Legacy manual batch start locked until plate entered:
- POST /api/batch/start manual branch returns 400 missing_plate when
  plate empty (strip + uppercase, no format regex)
- operator + monitoring start modals gain required plate input,
  confirm button disabled until filled
- operator plate tile also shows for manual batches (was do_manual only)
- do_manual unchanged (plate from DOs)

templates/operator.html

@@ -118,6 +118,7 @@
     .btn-disabled {
         opacity: 0.6;
         cursor: not-allowed;
+        pointer-events: none;
     }
 
     /* Simple Confirmation Modal */
@@ -425,10 +426,16 @@
     <div class="modal-box">
         <h3 class="modal-title"><i class="fa-solid fa-play" style="color: var(--accent-green);"></i> Mulai Batch Baru?</h3>
         <p class="modal-desc" id="startModalDesc">Pastikan truk sudah siap di posisi pemuatan karung.</p>
+        <div id="startPlateRow" style="display:none;margin:8px 0;">
+            <label for="startPlateInput" style="display:block;font-size:12px;color:var(--text-secondary);margin-bottom:4px;">Plat Nomor (wajib)</label>
+            <input type="text" id="startPlateInput" placeholder="B 1234 XYZ" autocomplete="off"
+                   style="width:100%;padding:8px 10px;border-radius:var(--radius-md);border:1px solid var(--border-color);background:var(--bg-card);color:var(--text-primary);font-size:14px;text-transform:uppercase;box-sizing:border-box;"
+                   oninput="onStartPlateInput()">
+        </div>
         <div id="startGateList" class="modal-desc" style="display:none;color:var(--accent-red);"></div>
         <div class="modal-actions">
             <button class="btn btn-secondary" onclick="closeModal('modalStart')">Batal</button>
-            <button class="btn btn-primary" style="background-color: var(--accent-green);" onclick="executeStartBatch()">Ya, Mulai</button>
+            <button id="btnStartConfirm" class="btn btn-primary" style="background-color: var(--accent-green);" onclick="executeStartBatch()">Ya, Mulai</button>
         </div>
     </div>
 </div>
@@ -687,10 +694,19 @@
         return (data && data.items) || [];
     }
 
+    function onStartPlateInput() {
+        const plate = document.getElementById('startPlateInput').value.trim();
+        const btn = document.getElementById('btnStartConfirm');
+        btn.classList.toggle('btn-disabled', plate.length === 0);
+    }
+
     function confirmStartBatch() {
         const gate = document.getElementById('startGateList');
         const desc = document.getElementById('startModalDesc');
+        const plateRow = document.getElementById('startPlateRow');
         if (currentMode === 'do_manual') {
+            plateRow.style.display = 'none';
+            document.getElementById('btnStartConfirm').classList.remove('btn-disabled');
             fetchStagedForStart().then(items => {
                 stagedDos = items;
                 const problems = [];
@@ -717,6 +733,9 @@
         }
         gate.style.display = 'none';
         desc.textContent = 'Pastikan truk sudah siap di posisi pemuatan karung.';
+        document.getElementById('startPlateInput').value = '';
+        plateRow.style.display = 'block';
+        onStartPlateInput();
         document.getElementById('modalStart').style.display = 'flex';
     }
 
@@ -744,6 +763,14 @@
             if (currentMode === 'do_manual') {
                 const items = await fetchStagedForStart();
                 body = JSON.stringify({ do_ids: items.map(d => d.id) });
+            } else if (currentMode === 'manual') {
+                const plate = document.getElementById('startPlateInput').value.trim().toUpperCase();
+                if (!plate) {
+                    closeModal('modalStart');
+                    alert('Plat nomor wajib diisi sebelum batch dimulai.');
+                    return;
+                }
+                body = JSON.stringify({ plate: plate });
             }
             const res = await fetch('/api/batch/start', {
                 method: 'POST',
@@ -817,7 +844,8 @@
             const plateItem = document.getElementById('opPlateItem');
             const plateDos = document.getElementById('opPlateDos');
             const batchActive = !!(data.success && data.batch_number);
-            const showPlate = currentMode === 'do_manual' && batchActive;
+            const showPlate = batchActive &&
+                (currentMode === 'do_manual' || !!(data.plate && data.plate.trim()));
             if (doSack) doSack.textContent = data.expected_sack || 0;
             if (doBox) doBox.textContent = data.expected_box || 0;
             if (plateItem) plateItem.style.display = showPlate ? 'block' : 'none';