## Why
### Problem
Users viewing a budget had no way to look back at historical periods —
the show page always displayed the current period with no navigation.
The cashflow page's period selector also used a different visual style
(loose buttons with gaps) compared to the new grouped button pattern
established for budgets.
## What
### Changes
- **Budget period navigation**: new `BudgetPeriodNavigation` component
renders a `ButtonGroup` with `[<] [date range] [>]` buttons. Clicking
the label returns to the current period; arrows navigate between
existing periods.
- **Backend**: `BudgetController::show` accepts an optional
`?period=<uuid>` query param to serve a specific period. Future periods
are blocked at the query level on both direct access and `nextPeriod`
resolution.
- **Dropdown**: removed the split `ButtonGroup` from the budget header;
"Edit budget" and "Delete budget" now live together in a single `˅`
dropdown.
- **Cashflow period selector**: updated `PeriodNavigation` to use
`ButtonGroup` + `size="icon"` buttons to match the same visual style.
- **Tests**: 4 new feature tests covering period navigation,
future-period blocking, cross-budget access (404), and `nextPeriod`
boundary behaviour.
## Verification
<img width="921" height="683" alt="image"
src="https://github.com/user-attachments/assets/ceb5f70b-a15a-4a36-ae49-5d84054a62f9"
/>
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