feat(fitness): download GPX from history detail

Export each cardio exercise's stored GPS track from the history
detail page. Cadence is emitted per-point via Garmin's
TrackPointExtension v1 so Strava/Garmin Connect preserve it.
Filename: YYYY-MM-DD-<workout> <mins>min <Activity>.gpx.

src/routes/api/fitness/sessions/[id]/gpx/+server.ts

@@ -5,6 +5,7 @@ import { WorkoutSession } from '$models/WorkoutSession';
 import type { IGpsPoint } from '$models/WorkoutSession';
 import { simplifyTrack } from '$lib/server/simplifyTrack';
 import { computeSessionKcal } from '$lib/server/computeSessionKcal';
+import { generateGpx, buildGpxFilename } from '$lib/server/gpxExport';
 import mongoose from 'mongoose';
 
 /** Haversine distance in km between two points */
@@ -56,6 +57,62 @@ function parseGpx(xml: string): IGpsPoint[] {
 	return points;
 }
 
+// GET /api/fitness/sessions/[id]/gpx?exerciseIdx=N — download GPX export of the track
+export const GET: RequestHandler = async ({ params, url, locals }) => {
+	const session = await locals.auth();
+	if (!session || !session.user?.nickname) {
+		return json({ error: 'Unauthorized' }, { status: 401 });
+	}
+
+	if (!mongoose.Types.ObjectId.isValid(params.id)) {
+		return json({ error: 'Invalid session ID' }, { status: 400 });
+	}
+
+	const exerciseIdx = parseInt(url.searchParams.get('exerciseIdx') ?? '', 10);
+	if (isNaN(exerciseIdx) || exerciseIdx < 0) {
+		return json({ error: 'Invalid exercise index' }, { status: 400 });
+	}
+
+	await dbConnect();
+
+	const workoutSession = await WorkoutSession.findOne({
+		_id: params.id,
+		createdBy: session.user.nickname
+	}).lean();
+
+	if (!workoutSession) {
+		return json({ error: 'Session not found' }, { status: 404 });
+	}
+
+	const ex = workoutSession.exercises[exerciseIdx];
+	if (!ex) {
+		return json({ error: 'Exercise index out of range' }, { status: 400 });
+	}
+	if (!ex.gpsTrack || ex.gpsTrack.length === 0) {
+		return json({ error: 'No GPS track on this exercise' }, { status: 404 });
+	}
+
+	const trackName = `${workoutSession.name} — ${ex.name}`;
+	const trackMs = ex.gpsTrack[ex.gpsTrack.length - 1].timestamp - ex.gpsTrack[0].timestamp;
+	const durationMin = trackMs > 0 ? trackMs / 60000 : (workoutSession.duration ?? 0);
+	const filename = buildGpxFilename({
+		startTime: workoutSession.startTime,
+		workoutName: workoutSession.name,
+		durationMin,
+		activityType: workoutSession.activityType,
+		fallbackActivity: ex.name
+	});
+	const xml = generateGpx(ex.gpsTrack, trackName);
+
+	return new Response(xml, {
+		status: 200,
+		headers: {
+			'Content-Type': 'application/gpx+xml; charset=utf-8',
+			'Content-Disposition': `attachment; filename="${filename}"; filename*=UTF-8''${encodeURIComponent(filename)}`
+		}
+	});
+};
+
 // POST /api/fitness/sessions/[id]/gpx — upload GPX file for an exercise
 export const POST: RequestHandler = async ({ params, request, locals }) => {
 	const session = await locals.auth();

src/routes/fitness/[history=fitnessHistory]/[id]/+page.svelte

@@ -1,7 +1,7 @@
 <script>
 	import { goto, invalidateAll } from '$app/navigation';
 	import { page } from '$app/stores';
-	import { Clock, Weight, Trophy, Trash2, Pencil, Plus, Upload, Route, X, RefreshCw, Gauge, Flame, Info, Mountain } from '@lucide/svelte';
+	import { Clock, Weight, Trophy, Trash2, Pencil, Plus, Upload, Download, Route, X, RefreshCw, Gauge, Flame, Info, Mountain } 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';
@@ -543,6 +543,11 @@
 		input.click();
 	}
 
+	/** @param {number} exIdx */
+	function downloadGpx(exIdx) {
+		window.location.href = `/api/fitness/sessions/${session._id}/gpx?exerciseIdx=${exIdx}`;
+	}
+
 	/** @param {number} exIdx */
 	async function removeGpx(exIdx) {
 		if (!await confirm(t('remove_gps_confirm', lang))) return;
@@ -836,6 +841,10 @@
 							</div>
 						{/if}
 						{/if}
+						<button class="gpx-download-btn" onclick={() => downloadGpx(exIdx)}>
+							<Download size={14} />
+							{t('download_gpx', lang)}
+						</button>
 					</div>
 				{:else if isCardio(ex.exerciseId)}
 					<button class="gpx-upload-btn" onclick={() => uploadGpx(exIdx)} disabled={uploading === exIdx}>
@@ -1345,6 +1354,24 @@
 		opacity: 0.5;
 		cursor: not-allowed;
 	}
+	.gpx-download-btn {
+		display: inline-flex;
+		align-items: center;
+		gap: 0.4rem;
+		margin-top: 0.75rem;
+		padding: 0.4rem 0.75rem;
+		background: transparent;
+		border: 1px solid var(--color-border);
+		border-radius: var(--radius-md);
+		color: var(--color-text-secondary);
+		font-size: 0.8rem;
+		cursor: pointer;
+		transition: border-color 0.15s, color 0.15s;
+	}
+	.gpx-download-btn:hover {
+		border-color: var(--color-primary);
+		color: var(--color-primary);
+	}
 
 	/* GPS charts */
 	.chart-section {