feat: Phase 4 — Live, Vitals, Zones, MAT, Settings screens

LiveScreen: GaussianSplatWebView + gaussian-splats.html (Three.js 3D viz), LiveHUD
VitalsScreen: BreathingGauge, HeartRateGauge, MetricCard (Reanimated arcs)
ZonesScreen: FloorPlanSvg (SVG heatmap 20x20), ZoneLegend, useOccupancyGrid
MATScreen: MatWebView + mat-dashboard.html (pure-JS disaster response), AlertCard/List, SurvivorCounter
SettingsScreen: ServerUrlInput (URL validation + test), ThemePicker, RssiToggle

Verified: tsc 0 errors, jest passes

mobile/src/screens/VitalsScreen/BreathingGauge.tsx

@@ -0,0 +1,63 @@
+import { useMemo } from 'react';
+import { View, StyleSheet } from 'react-native';
+import { usePoseStore } from '@/stores/poseStore';
+import { GaugeArc } from '@/components/GaugeArc';
+import { colors } from '@/theme/colors';
+import { ThemedText } from '@/components/ThemedText';
+
+const BREATHING_MIN_BPM = 0;
+const BREATHING_MAX_BPM = 30;
+const BREATHING_BAND_MAX = 0.3;
+
+const clamp = (value: number, min: number, max: number) => Math.max(min, Math.min(max, value));
+
+const deriveBreathingValue = (
+  breathingBand?: number,
+  breathingBpm?: number,
+): number => {
+  if (typeof breathingBpm === 'number' && Number.isFinite(breathingBpm)) {
+    return clamp(breathingBpm, BREATHING_MIN_BPM, BREATHING_MAX_BPM);
+  }
+
+  const bandValue = typeof breathingBand === 'number' && Number.isFinite(breathingBand) ? breathingBand : 0;
+  const normalized = clamp(bandValue / BREATHING_BAND_MAX, 0, 1);
+  return normalized * BREATHING_MAX_BPM;
+};
+
+export const BreathingGauge = () => {
+  const breathingBand = usePoseStore((state) => state.features?.breathing_band_power);
+  const breathingBpm = usePoseStore((state) => state.lastFrame?.vital_signs?.breathing_bpm);
+
+  const value = useMemo(
+    () => deriveBreathingValue(breathingBand, breathingBpm),
+    [breathingBand, breathingBpm],
+  );
+
+  return (
+    <View style={styles.container}>
+      <ThemedText preset="labelMd" style={styles.label}>
+        BREATHING
+      </ThemedText>
+      <GaugeArc value={value} min={BREATHING_MIN_BPM} max={BREATHING_MAX_BPM} label="" unit="BPM" color={colors.accent} />
+      <ThemedText preset="labelMd" color="textSecondary" style={styles.unit}>
+        BPM
+      </ThemedText>
+    </View>
+  );
+};
+
+const styles = StyleSheet.create({
+  container: {
+    alignItems: 'center',
+    justifyContent: 'center',
+    gap: 6,
+  },
+  label: {
+    color: '#94A3B8',
+    letterSpacing: 1,
+  },
+  unit: {
+    marginTop: -12,
+    marginBottom: 4,
+  },
+});