feat(stammbaum): drive viewBox from PanZoomState (pan + zoom) (#692)

Replace the scalar zoom prop with a {x,y,z} PanZoomState. The viewBox centre
is offset by the pan and width/height scaled by zoom; the default {0,0,1}
frames the whole tree (fit-to-screen). Page header buttons now step view.z
through clampZoom over the resolved 0.25–3.0 range.

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

frontend/src/lib/person/genealogy/panZoom.ts

@@ -17,6 +17,9 @@ export const DEFAULT_ZOOM = 1;
 /** Minimum zoom a recentre will snap up to so the focal node's text is legible (OQ-005). */
 export const LEGIBLE_ZOOM = 1;
 
+/** Fixed zoom increment per keyboard `+`/`-` press and per control-button click (OQ-002). */
+export const ZOOM_STEP_KB = 0.1;
+
 /**
  * The canvas view state. `x`/`y` are pan offsets applied to the viewBox centre
  * (SVG user units); `z` is the zoom factor. The default `{0, 0, 1}` frames the