feat(stammbaum): centre the tapped person above the bottom sheet (#703)

On a touch viewport (below the md breakpoint, where the bottom sheet
overlays the lower part of the canvas), tapping a person now auto-centres
them via recentreAbove with a 0.3 height bias, so the highlighted anchor
lands in the band above the sheet instead of behind it (AC8). On desktop
the side panel is a flex sibling that never covers the tree, so the bias
is 0 and selection does not pan. StammbaumTree's recentre effect takes a
centreBiasFraction prop and the page drives it from a matchMedia flag.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumTree.svelte

@@ -13,7 +13,7 @@ import {
 	type PanZoomState,
 	clampZoom,
 	clampPan,
-	recentreOn,
+	recentreAbove,
 	cornerView,
 	ZOOM_STEP_KB
 } from '$lib/person/genealogy/panZoom';
@@ -35,6 +35,11 @@ interface Props {
 	onPanZoom?: (state: PanZoomState) => void;
 	/** When set to a node id, the canvas recentres on that node (US-PAN-005). */
 	centreOnId?: string | null;
+	/**
+	 * Fraction of the viewBox height to lift a recentred node above the centre,
+	 * so on a phone the anchor clears the bottom sheet (#703 AC8). 0 centres it.
+	 */
+	centreBiasFraction?: number;
 	/** Fired on the first pointer interaction with the canvas (affordance dismiss). */
 	onActivity?: () => void;
 	/** When true, the initial view is anchored to the tree's top-left corner. */
@@ -56,6 +61,7 @@ let {
 	panZoom,
 	onPanZoom = () => {},
 	centreOnId = null,
+	centreBiasFraction = 0,
 	onActivity,
 	anchorTopLeft = false,
 	onSelect,
@@ -219,7 +225,7 @@ $effect(() => {
 	if (!id) return;
 	untrack(() => {
 		const c = nodeCenter(id);
-		if (c) onPanZoom(recentreOn(c, baseCentre, panZoom, true));
+		if (c) onPanZoom(recentreAbove(c, baseCentre, panZoom, baseDims.h, centreBiasFraction));
 	});
 });