[ladder-editor] P2: React Router v7 future flag warnings

[Mikecranesync]

Two React Router v6→v7 migration warnings appear on every load:

1. v7_startTransition — state updates not wrapped in React.startTransition
2. v7_relativeSplatPath — relative route resolution changing in v7

Fix: Add future flags to the Router:

<BrowserRouter future={{ v7_startTransition: true, v7_relativeSplatPath: true }}>

Low priority — warnings only, no functional impact.