fix: correct monitoring flag and battery/memory offsets in _decode_monitor_status

section[6] is the monitoring flag (was wrongly section[1] — section[1] is always
0x00 in both states). Battery and memory fields use relative-from-end offsets
(section[-11:-9], section[-9:-5], section[-5:-1]) instead of absolute positions,
which broke when the payload grew by 3 bytes in monitoring mode.

Confirmed from full byte diff of 142 0xE3 frames in 4-8-26/2ndtry capture.
SFM start_monitoring now polls /device/monitor/status every 5s for up to 60s
instead of a fixed 25s delay (unit runs ~40s on-device sensor check before
confirming monitoring state).

Also corrects stale 1C→6E response anomaly claim in protocol reference — no
exceptions to the 0xFF−SUB rule are known.

sfm/sfm_webapp.html

@@ -813,13 +813,14 @@ function populateDeviceBar() {
 
 // ── Monitoring ─────────────────────────────────────────────────────────────────
 async function refreshMonitorStatus() {
-  if (!devHost()) return;
+  if (!devHost()) return null;
   try {
     const r = await fetch(`${api()}/device/monitor/status?${deviceParams()}`);
-    if (!r.ok) return;
+    if (!r.ok) return null;
     const s = await r.json();
     updateMonitorPanel(s);
-  } catch (_) {}
+    return s;
+  } catch (_) { return null; }
 }
 
 function updateMonitorPanel(s) {
@@ -832,24 +833,23 @@ function updateMonitorPanel(s) {
   const stopB  = document.getElementById('mon-stop-btn');
 
   if (s.is_monitoring) {
-    badge.textContent  = 'MONITORING';
-    badge.className    = 'mon-status-badge monitoring';
-    panel.className    = 'monitoring';
-    startB.disabled    = true;
-    stopB.disabled     = false;
-    batEl.textContent  = '—';
-    memTEl.textContent = '—';
-    memFEl.textContent = '—';
+    badge.textContent = 'MONITORING';
+    badge.className   = 'mon-status-badge monitoring';
+    panel.className   = 'monitoring';
+    startB.disabled   = true;
+    stopB.disabled    = false;
   } else {
-    badge.textContent  = 'IDLE';
-    badge.className    = 'mon-status-badge idle';
-    panel.className    = 'idle';
-    startB.disabled    = false;
-    stopB.disabled     = true;
-    batEl.textContent  = s.battery_v != null ? `${s.battery_v.toFixed(2)} V` : '—';
-    memTEl.textContent = s.memory_total_kb != null ? `${s.memory_total_kb} KB` : '—';
-    memFEl.textContent = s.memory_free_kb  != null ? `${s.memory_free_kb} KB`  : '—';
+    badge.textContent = 'IDLE';
+    badge.className   = 'mon-status-badge idle';
+    panel.className   = 'idle';
+    startB.disabled   = false;
+    stopB.disabled    = true;
   }
+  // Battery and memory are available in both states — update if present,
+  // keep previous value if this was an optimistic update with no real data.
+  if (s.battery_v != null)       batEl.textContent  = `${s.battery_v.toFixed(2)} V`;
+  if (s.memory_total_kb != null) memTEl.textContent = `${s.memory_total_kb} KB`;
+  if (s.memory_free_kb  != null) memFEl.textContent = `${s.memory_free_kb} KB`;
 }
 
 async function startMonitoring() {
@@ -860,13 +860,47 @@ async function startMonitoring() {
   try {
     const r = await fetch(`${api()}/device/monitor/start?${deviceParams()}`, { method: 'POST' });
     if (!r.ok) { const e = await r.json().catch(() => ({})); throw new Error(e.detail || r.statusText); }
-    setStatus('Monitoring started.', 'ok');
-    await refreshMonitorStatus();
+
+    // Optimistically show MONITORING immediately. The unit may run a ~40s on-device
+    // sensor check before fully entering monitor mode. We poll status every 5s for
+    // up to 60s, updating the badge when is_monitoring flips to true.
+    updateMonitorPanel({ is_monitoring: true });
+    setStatus('Monitoring started — sensor check in progress (~40s)…', 'loading');
+    btn.textContent = '▶ Start';
+
+    _pollMonitorConfirm(0);
+
   } catch (e) {
     setStatus(`Start monitoring failed: ${e.message}`, 'error');
     btn.disabled = false;
+    btn.textContent = '▶ Start';
+  }
+}
+
+async function _pollMonitorConfirm(attempt) {
+  // Poll /device/monitor/status every 5s for up to 60s after startMonitoring().
+  // Updates the panel on each successful poll. Resolves once is_monitoring is true
+  // or after 12 attempts (60s), whichever comes first.
+  const MAX_ATTEMPTS = 12;
+  const INTERVAL_MS  = 5000;
+  if (attempt >= MAX_ATTEMPTS) {
+    const s = await refreshMonitorStatus();
+    if (!s || !s.is_monitoring) {
+      setStatus('Warning: unit did not confirm monitoring state after 60s. Check device.', 'error');
+    } else {
+      setStatus('Monitoring active.', 'ok');
+    }
+    return;
+  }
+  await new Promise(res => setTimeout(res, INTERVAL_MS));
+  const s = await refreshMonitorStatus();
+  if (s && s.is_monitoring) {
+    setStatus('Monitoring active.', 'ok');
+  } else {
+    const elapsed = (attempt + 1) * 5;
+    setStatus(`Sensor check in progress… (${elapsed}s elapsed)`, 'loading');
+    _pollMonitorConfirm(attempt + 1);
   }
-  btn.textContent = '▶ Start';
 }
 
 async function stopMonitoring() {