feat(devices): add Health tab, log explorer, and fleet health indicators

Surfaces the new device-health telemetry (boot events, ping RTT,
diagnostics reports, alert events) across the console:

- HealthTab: boot/diagnostics timeline, CPU temp and RSSI charts
  (LineChart), current alert status, and a Settings sub-tab for
  DeviceHealthSettings thresholds
- LogsTab: dedicated log explorer embedded in the Health tab, with
  level/source filtering and fmtLogTimestamp for dense timestamp rows
- OverviewTab: a "Latest Device Issue" card showing the most recent
  alert event, colour-coded by severity and fading with age, plus a
  modal to inspect the surrounding log lines
- DeviceList/DeviceListCardView/DeviceListMapView: fleet list gains a
  SignalIndicator-based RSSI display in place of the plain online dot

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/bellcloud/devices/DeviceListMapView.jsx

@@ -589,7 +589,7 @@ export default function DeviceListMapView({ devices, loading, mqttStatusMap, onV
                   onClick={(e) => { e.stopPropagation(); selectDevice(device) }}
                   style={{ cursor: 'pointer' }}
                   role="button"
-                  aria-label={`${device.device_name || 'Device'} — ${color.label}`}
+                  aria-label={`${device.console_name || device.device_name || 'Device'} — ${color.label}`}
                 >
                   {/* Breathing ring */}
                   <circle
@@ -767,7 +767,7 @@ export default function DeviceListMapView({ devices, loading, mqttStatusMap, onV
                     aria-hidden="true"
                   />
                   <h2 className="dmv-drawer__name">
-                    {selected.device_name || 'Unnamed Device'}
+                    {selected.console_name || selected.device_name || 'Unnamed Device'}
                   </h2>
                   <button
                     className="dmv-drawer__close"