## Summary
- Fixes
[PHP-LARAVEL-9](https://whisper-money.sentry.io/issues/PHP-LARAVEL-9):
`ReferenceError: window is not defined` during Inertia SSR of
`CashflowPage`.
- `CashflowController` now reads + validates the `period` query param
(`YYYY-MM`) and passes it as an Inertia prop.
- Client `useState` initializer reads from `usePage().props.period`
instead of `window.location.search`, making SSR safe and avoiding
hydration mismatch.
- `useEffect` URL sync now compares against the prop, not
`window.location.search`.
## Why (c) not a `typeof window` guard
Server-provided params avoid hydration mismatch and keep URL as single
source of truth.
## Tests
New `tests/Feature/CashflowPageTest.php`:
- guest redirect
- no query param → `period` prop `null`
- valid `?period=2025-03` → prop `'2025-03'`
- invalid value → `null`
- malformed format (`2025-3`) → `null`
All 5 pass (50 assertions).
## Scope
Cashflow only. Other pages with similar `window.location.search` in
`useState` initializers (`onboarding/index.tsx`, `welcome.tsx`,
`auth/login.tsx`) left for follow-up.
Fixes PHP-LARAVEL-9
22952c4e75