fix(memory-graph): center arrow-key navigation in the visible graph area

[abhay-codes07]

Problem

viewport.centerOn(worldX, worldY, width, height) positions the target point at height / 2 on screen:

this.targetPanY = height / 2 - worldY * this.zoom

The component derives a separate graphFitHeight that, on compact viewports, reserves room for the bottom control bar:

const graphFitHeight = isCompactViewport
  ? Math.max(containerSize.height - 170, 240)
  : containerSize.height

Auto-fit, the center button, and the zoom controls all pass graphFitHeight, so they place content in the middle of the visible graph area. But selectAndCenter (arrow-key node navigation) passed the full containerSize.height:

viewportRef.current.centerOn(n.x, n.y, containerSize.width, containerSize.height)

So on mobile, stepping through nodes with the keyboard centers the selected node at the midpoint of the whole container, which puts it partly behind the bottom controls, while the dedicated center button drops the same graph correctly above them.

Fix

Pass graphFitHeight (and depend on it) so selectAndCenter lands the node where the other centering paths already do:

viewportRef.current.centerOn(n.x, n.y, containerSize.width, graphFitHeight)

On desktop graphFitHeight === containerSize.height, so nothing changes there. Full memory-graph suite (188 tests) passes and tsc --noEmit is clean.

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