Initial implementation: Bordanlage boat onboard system

Complete multiroom audio + navigation dashboard:
- Docker stack: SignalK, Snapcast (4 zones), librespot, shairport-sync, Mopidy, Jellyfin, Portainer
- React 18 + Vite dashboard with nautical dark theme
- Full mock system (SignalK NMEA simulation, Snapcast zones, Mopidy player)
- Real API clients for all services with reconnect logic
- SVG instruments: Compass, WindRose, Gauge, DepthSounder, SpeedLog
- Pages: Overview, Navigation, Audio (zones/radio/library), Systems
- Dev mode runs fully without hardware (make dev)

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

bordanlage/dashboard/src/components/instruments/DepthSounder.jsx

@@ -0,0 +1,16 @@
+// Depth sounder with alarm threshold.
+import Gauge from './Gauge.jsx'
+
+export default function DepthSounder({ depth }) {
+  return (
+    <Gauge
+      value={depth}
+      min={0}
+      max={30}
+      label="Depth"
+      unit="m"
+      warning={3}
+      danger={1.5}
+    />
+  )
+}