fix(client): guard observationTabOpen against a missing right-column tree

DSH 0.1.5 (with better-sidebar 0.19+) moved the right column to dsh native Sidebar, so the sidebar snapshot no longer carries `splits`. observationTabOpen still walked [state.splits, state.bottomSplits] and fed each root straight into leafNodes(), which dereferences node.kind - with `splits` absent that throws "TypeError: Cannot read properties of undefined (reading kind)" on every render.

Because the observation tab registers inside better-sidebar render tree, the error surfaces under better-sidebar error boundary as a persistent red banner ("dsh-better-sidebar: Cannot read properties of undefined (reading kind)") even though the faulty walk lives here. Mark `splits` optional and skip absent roots before walking.

Verified on dsh 0.1.5-rc.2 + better-sidebar 0.19.0-alpha.1: with this guard in the built client bundle, the banner and the console TypeError are gone and observation tab detection behaves as before.