refactor(ui): split IntensitySlider pieces

Extract the slider track, levels, card, and modal into\nfocused subcomponents with shared styles/constants.\nPreserve the existing IntensitySlider API.

apps/mobile/src/components/ui/IntensitySlider/IntensitySliderTrack.tsx

@@ -0,0 +1,30 @@
+import React from 'react';
+import { View, StyleSheet } from 'react-native';
+import { LinearGradient } from 'expo-linear-gradient';
+import Animated, { type AnimatedStyleProp } from 'react-native-reanimated';
+import type { ViewStyle } from 'react-native';
+import type { IntensityLevel } from '@/types';
+import { HEAT_COLORS } from './constants';
+import { styles } from './styles';
+
+interface IntensitySliderTrackProps {
+    value: IntensityLevel;
+    progressStyle: AnimatedStyleProp<ViewStyle>;
+}
+
+export function IntensitySliderTrack({ value, progressStyle }: IntensitySliderTrackProps) {
+    return (
+        <View style={styles.trackContainer}>
+            <View style={styles.track}>
+                <Animated.View style={[styles.progressFill, progressStyle]}>
+                    <LinearGradient
+                        colors={['#9b59b6', HEAT_COLORS[value - 1]]}
+                        start={{ x: 0, y: 0 }}
+                        end={{ x: 1, y: 0 }}
+                        style={StyleSheet.absoluteFill}
+                    />
+                </Animated.View>
+            </View>
+        </View>
+    );
+}