feat(fitness/stats): 4 cm minimum y-axis range on body-part history charts

Body-measurement variation of <4 cm used to stretch the full chart
height, making normal weekly noise look dramatic. Now the y-axis
enforces a 4 cm floor centered on the data's midpoint; wider swings
render at their actual range as before.

- FitnessChart: new optional `yMin` / `yMax` props mapped to Chart.js
  `suggestedMin` / `suggestedMax` — soft bounds, so data that exceeds
  them still widens the axis.
- `/fitness/stats/history/[part]`: computes min/max across available
  values (both sides if paired), enforces the 4 cm floor, passes to
  FitnessChart. Tick distance stays on Chart.js auto — small ranges
  get 0.5 cm ticks, wider ones scale up naturally.

src/lib/components/fitness/FitnessChart.svelte

@@ -11,10 +11,12 @@
 	 *   height?: string,
 	 *   yUnit?: string,
 	 *   goalLine?: number,
-	 *   tooltipFormatter?: (value: number, datasetIndex: number, dataIndex: number, label: string) => string
+	 *   tooltipFormatter?: (value: number, datasetIndex: number, dataIndex: number, label: string) => string,
+	 *   yMin?: number,
+	 *   yMax?: number
 	 * }}
 	 */
-	let { type = 'line', data, title = '', height = '250px', yUnit = '', goalLine = undefined, tooltipFormatter = undefined } = $props();
+	let { type = 'line', data, title = '', height = '250px', yUnit = '', goalLine = undefined, tooltipFormatter = undefined, yMin = undefined, yMax = undefined } = $props();
 
 	/** @type {HTMLCanvasElement | undefined} */
 	let canvas = $state(undefined);
@@ -125,6 +127,8 @@
 					},
 					y: {
 						beginAtZero: type === 'bar',
+						suggestedMin: yMin,
+						suggestedMax: yMax,
 						grid: { color: gridColor },
 						border: { display: false },
 						ticks: {