Desktop and mobile share one hook and one menu. `useFutureBufferMode`
reads `__global!future-buffer-start` and the month's `frozen` cell
through sheet bindings, so a mode change from another device shows up
without a reload, and `isManagedMonth` is a pure rule.
- To Budget menu item "For next month: Automatic…" / "Manual…" opening
the mode modal. Shown with the experimental flag, and always once the
mode is on.
- Mode modal with the Manual and Automatic choices, the start month, and
the consequences of switching either way.
- Managed months hide Hold, Reset, Disable current auto hold, the income
auto-hold toggle and the rollover arrow, explain "For next month" in a
tooltip, and say how much is held for future months under
"Overbudgeted". Frozen months keep Hold and Reset and lose the
auto-hold controls.
- The budget-type switch says automatic mode is turned off first.
- `useSheetValue` observes the sheet a binding names, which the global
cell needs.
- `month` reaches `ToBudgetAmount`, `TotalsList` and `BudgetSummary`,
`TotalsList` provides its own sheet, and `monthFromSheet` maps a sheet
back to its month.
Co-authored-by: Ngoc Pham <ngocphamm@fastmail.fm>