feat(stammbaum): keyboard pan/zoom on the canvas (#692)

+/- zoom by the fixed step and arrow keys pan by a tenth of the visible
extent, emitted via onPanZoom. Provides the keyboard-only alternative path
required by NFR-A11Y-002. Nodes keep their own Enter/Space selection.

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

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

@@ -104,6 +104,7 @@ function zoomOut() {
 					edges={data.edges}
 					selectedId={selectedId}
 					panZoom={view}
+					onPanZoom={(v) => (view = v)}
 					onSelect={(id) => (selectedId = id)}
 				/>
 			</div>