fitness: improve workouts chart with goal line, max bar width, and full 10-week range

Add weekly goal as a solid horizontal line on the bar chart via a
custom Chart.js plugin. Cap bar width at 40px. Always show all 10
weeks including empty ones instead of trimming leading zeros.

src/routes/fitness/[stats=fitnessStats]/+page.svelte

@@ -187,6 +187,7 @@
 					data={workoutsChartData}
 					title={t('workouts_per_week', lang)}
 					height="220px"
+					goalLine={goalWeekly ?? undefined}
 				/>
 			{:else}
 				<p class="empty-chart">{t('no_workout_data', lang)}</p>