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/monitoring.html

@@ -405,10 +405,16 @@
 <div id="modalStart" style="position: fixed; inset: 0; background: rgba(0,0,0,0.5); display: none; align-items: center; justify-content: center; z-index: 999; padding: 20px;">
     <div style="background: var(--bg-card); border: 1px solid var(--border-color); border-radius: var(--radius-lg); padding: 24px; max-width: 400px; width: 100%; text-align: center;">
         <h3 style="font-size: 18px; font-weight: 700; margin-bottom: 12px;"><i class="fa-solid fa-play" style="color: var(--accent-green);"></i> Mulai Batch Baru?</h3>
-        <p style="font-size: 14px; color: var(--text-secondary); margin-bottom: 24px;">Counter akan diaktifkan dan batch baru akan dibuat.</p>
+        <p style="font-size: 14px; color: var(--text-secondary); margin-bottom: 12px;">Counter akan diaktifkan dan batch baru akan dibuat.</p>
+        <div id="monStartPlateRow" style="display:none;margin-bottom: 24px; text-align: left;">
+            <label for="monStartPlateInput" style="display:block;font-size:12px;color:var(--text-secondary);margin-bottom:4px;">Plat Nomor (wajib)</label>
+            <input type="text" id="monStartPlateInput" placeholder="B 1234 XYZ" autocomplete="off"
+                   oninput="onMonStartPlateInput()"
+                   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;">
+        </div>
         <div style="display: flex; gap: 12px;">
             <button class="btn btn-secondary" style="flex: 1;" onclick="document.getElementById('modalStart').style.display='none'">Batal</button>
-            <button class="btn btn-primary" style="flex: 1; background-color: var(--accent-green);" onclick="executeStartBatch()">Ya, Mulai</button>
+            <button id="monBtnStartConfirm" class="btn btn-primary" style="flex: 1; background-color: var(--accent-green);" onclick="executeStartBatch()">Ya, Mulai</button>
         </div>
     </div>
 </div>
@@ -479,7 +485,21 @@
         }
     }
 
+    function onMonStartPlateInput() {
+        const plate = document.getElementById('monStartPlateInput').value.trim();
+        const btn = document.getElementById('monBtnStartConfirm');
+        btn.style.opacity = plate.length === 0 ? '0.6' : '';
+        btn.style.pointerEvents = plate.length === 0 ? 'none' : '';
+    }
+
     function confirmStartBatch() {
+        const plateRow = document.getElementById('monStartPlateRow');
+        const isManual = currentBatchMode === 'manual';
+        plateRow.style.display = isManual ? 'block' : 'none';
+        if (isManual) {
+            document.getElementById('monStartPlateInput').value = '';
+            onMonStartPlateInput();
+        }
         document.getElementById('modalStart').style.display = 'flex';
     }
 
@@ -490,7 +510,18 @@
     async function executeStartBatch() {
         document.getElementById('modalStart').style.display = 'none';
         try {
-            const res = await fetch('/api/batch/start', { method: 'POST' });
+            let body = null;
+            let headers = {};
+            if (currentBatchMode === 'manual') {
+                const plate = document.getElementById('monStartPlateInput').value.trim().toUpperCase();
+                if (!plate) {
+                    alert('Plat nomor wajib diisi sebelum batch dimulai.');
+                    return;
+                }
+                body = JSON.stringify({ plate: plate });
+                headers = { 'Content-Type': 'application/json' };
+            }
+            const res = await fetch('/api/batch/start', { method: 'POST', headers: headers, body: body });
             const data = await res.json();
             if (data.success) {
                 await pollCurrentBatch();