fix(fitness): request ACTIVITY_RECOGNITION for cadence

Android step detector silently returns no events on API 29+
when ACTIVITY_RECOGNITION is ungranted, so cadence was always
absent from recorded tracks. Declare the permission, request
it at GPS start, guard sensor registration and retry it from
MainActivity.onRequestPermissionsResult when the user grants
mid-session, and toast a hint if they deny.

src/routes/fitness/[workout=fitnessWorkout]/[active=fitnessActive]/+page.svelte

@@ -4,6 +4,7 @@
 	import { Trash2, Play, Pause, Trophy, Clock, Dumbbell, Route, RefreshCw, Check, ChevronUp, ChevronDown, Flame, MapPin, Volume2, X, Timer, Plus, GripVertical, Repeat } from '@lucide/svelte';
 	import { detectFitnessLang, fitnessSlugs, t } from '$lib/js/fitnessI18n';
 	import { confirm } from '$lib/js/confirmDialog.svelte';
+	import { toast } from '$lib/js/toast.svelte';
 
 	const lang = $derived(detectFitnessLang($page.url.pathname));
 	const sl = $derived(fitnessSlugs(lang));
@@ -359,6 +360,20 @@
 		}
 	}
 
+	let _cadenceWarned = false;
+	/** After the user resolves the permission dialog, check if cadence will work.
+	 *  We wait a few seconds to let the system dialog settle before toasting. */
+	function maybeWarnCadence() {
+		if (_cadenceWarned) return;
+		setTimeout(() => {
+			if (_cadenceWarned) return;
+			if (!gps.cadenceAvailable()) {
+				_cadenceWarned = true;
+				toast.info(t('cadence_permission_missing', lang));
+			}
+		}, 4000);
+	}
+
 	let gpsToggling = $state(false);
 	async function toggleGps() {
 		if (gpsToggling) return;
@@ -369,6 +384,7 @@
 					useGps = true;
 				} else {
 					useGps = await gps.start(getVoiceGuidanceConfig());
+					if (useGps) maybeWarnCadence();
 				}
 			} else {
 				await gps.stop();
@@ -471,6 +487,7 @@
 		if (workout.mode === 'gps' && !gpsStarted && !gps.isTracking) {
 			_prestartGps = true;
 			gps.start(undefined, true);
+			maybeWarnCadence();
 		}
 	});
 
@@ -510,6 +527,7 @@
 		const exercise = getExerciseById(exerciseId);
 		if (exercise?.bodyPart === 'cardio' && gps.available && !useGps && !gps.isTracking) {
 			useGps = await gps.start(getVoiceGuidanceConfig());
+			if (useGps) maybeWarnCadence();
 		}
 	}
 
@@ -529,6 +547,7 @@
 				gpsStarted = true;
 				useGps = true;
 				workout.resumeTimer();
+				maybeWarnCadence();
 			}
 		} finally {
 			gpsStarting = false;