fitness: use time-scale x-axis for weight chart to handle date gaps

Weight chart now spaces data points proportionally to actual dates
instead of evenly. Days without a weight log no longer compress adjacent
points together. Uses Chart.js time scale with chartjs-adapter-date-fns.

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

@@ -82,6 +82,7 @@
 
 	const weightChartData = $derived({
 		labels: stats.weightChart?.labels ?? [],
+		dates: stats.weightChart?.dates,
 		datasets: [
 			...(hasSma ? [
 				{