feat: record cadence from step detector during GPS workouts

Use Android TYPE_STEP_DETECTOR sensor in LocationForegroundService to
count steps in a 15s rolling window. Cadence (spm) is computed at each
GPS point and stored alongside lat/lng/altitude/speed. Session detail
page shows cadence chart when data is available.

No additional permissions required — step detector is not a restricted
sensor. Gracefully skipped on devices without the sensor.

src/lib/js/fitnessI18n.ts

@@ -108,6 +108,8 @@ const translations: Translations = {
 	elevation_unit: { en: 'm', de: 'm' },
 	elevation_gain: { en: 'Gain', de: 'Anstieg' },
 	elevation_loss: { en: 'Loss', de: 'Abstieg' },
+	cadence: { en: 'Cadence', de: 'Kadenz' },
+	cadence_unit: { en: 'spm', de: 'spm' },
 	personal_records: { en: 'Personal Records', de: 'Persönliche Rekorde' },
 	delete_session_confirm: { en: 'Delete this workout session?', de: 'Dieses Training löschen?' },
 	remove_gps_confirm: { en: 'Remove GPS track from this exercise?', de: 'GPS-Track von dieser Übung entfernen?' },

src/lib/js/gps.svelte.ts

@@ -10,6 +10,7 @@ export interface GpsPoint {
 	lng: number;
 	altitude?: number;
 	speed?: number;
+	cadence?: number; // steps per minute, from step detector sensor
 	timestamp: number;
 }