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chore: update react router step 2: startTransition (#12119)

## Summary
- Part of the React Router v6 → v7 migration. Enables the
`v7_startTransition` future flag, which wraps router state updates in
`React.startTransition` so navigation can yield to more urgent work and
integrate with Suspense.
- Adds a new `reactRouter_v7_startTransition` UI flag 
- The test renderer enables the flag unconditionally so the suite always
exercises the v7 path.

## Why it should be safe
- Every `React.lazy(...)` call in the frontend is declared at module
scope (not recreated per render), which is the failure mode the
migration guide specifically calls out for `startTransition`.
- All lazy route components render below the top-level `<Suspense
fallback={<Loader type='fullscreen' />}>` in `App.tsx`, so any
suspension during a transition has a fallback to settle to.
- No `flushSync` or synchronous-state-after-navigate patterns in the
codebase.

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

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