feat(stammbaum): touch/mouse/wheel pan & pinch zoom gestures (#692)

Add a panZoomGestures action: one-finger/left-button drag pans, two-finger
pinch and Ctrl+wheel zoom around the centroid, plain wheel pans. Pan is
edge-clamped via clampPan (no infinite scroll), a real drag suppresses the
trailing node click, and inertia decays after release unless prefers-reduced-
motion. Canvas container switches from native scroll to overflow-hidden.

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

frontend/src/routes/stammbaum/+page.svelte

@@ -98,7 +98,7 @@ function zoomOut() {
 		</div>
 	{:else}
 		<div class="flex flex-1 overflow-hidden">
-			<div class="flex-1 overflow-auto bg-muted/20">
+			<div class="flex-1 overflow-hidden bg-muted/20">
 				<StammbaumTree
 					nodes={data.nodes}
 					edges={data.edges}