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/lib/person/genealogy/panZoom.ts

@@ -113,6 +113,20 @@ export function zoomAtPoint(
 	};
 }
 
+/**
+ * Clamp the pan offset so the canvas cannot be dragged off the edge (US-PAN-001
+ * AC4 — no infinite scroll). The pannable range on each axis is half the
+ * difference between the base frame and the (smaller) zoomed viewBox; when the
+ * whole tree fits (z ≤ 1) the range collapses to zero, so the view stays centred.
+ */
+export function clampPan(state: PanZoomState, baseW: number, baseH: number): PanZoomState {
+	const clampAxis = (pan: number, base: number) => {
+		const limit = Math.max(0, (base - base / state.z) / 2);
+		return Math.min(limit, Math.max(-limit, pan)) || 0; // normalise -0 → 0
+	};
+	return { x: clampAxis(state.x, baseW), y: clampAxis(state.y, baseH), z: state.z };
+}
+
 /**
  * Pan so a node sits at the viewBox centre (US-PAN-005). Because the viewBox
  * centre is `baseCentre + pan` independent of zoom, centring is a pure pan: