feat(money): one decimals setting for the dataset, not one per currency

`Settings.digits` replaces `CurrencyDef.digits`. Every amount rounds to it,
whatever its currency, and `digitsFor` is gone: the sixteen call sites read the
setting, and the domain functions that took a currency table purely to look
digits up take a `digits: number` instead. Schema 6.

The owner asked for this with the cost in front of them, and the cost is real:
a dataset mixing minor units cannot be described any more. Yen at 0 beside
kroner at 2 has no correct global value — 2 gives ¥1234.56, an amount yen
cannot represent; 0 rounds 49.95 DKK to 50 and drifts the carry from the bank.
Nothing in the app warns. docs/specs/2026-09-02-global-decimals-design.md
argues it in full, including what re-adding a per-currency override would take,
and AGENTS.md §1 now says not to reintroduce one without revisiting the
decision.

The 5 -> 6 step takes the MAXIMUM of the per-currency digits, not the base
currency's. A dataset holding a three-decimal currency already has
three-decimal amounts in it, and adopting the base's 2 would truncate the next
edit of one; the maximum can only keep more precision, never less. On every
dataset this app has written all four currencies record 2, so nothing rounds
differently than before.

Changing the setting does not rewrite stored amounts. 49.95 stays 49.95 and
shows as 50; totals recompute at the new precision. Rewriting each Money is a
destructive whole-dataset write, which §1 owes a backup export first, and it is
lossy one way.

Display turned out to be the hole in the plan. src/ui/format.ts pinned
minimumFractionDigits: 2 in a module-level Intl.NumberFormat, so it never read
the dataset at all — equally broken under per-currency digits, where a JPY
rendered "3,333.00", so a pre-existing gap rather than a regression. Shipping a
Settings control that display ignores would look broken at 0 and be lossy at 3,
hiding precision the dataset holds. So format.ts takes digits like roundMoney
does, useMoneyFormat() binds them for the 64 call sites, and postRowCells takes
the formatter as an argument because it is a plain helper, not a component.

currencyDigits.test.ts is rewritten rather than deleted. Driving a dataset at
ZERO decimals through rounding, both FX directions, both split modes, plan
division, the fold and the month view still tells a real implementation from
one assuming 2 — it just cannot prove per-currency flow, because there is none.
It also asserts that raising the setting to 2 changes what the fold produces.

Verified in Chrome, not only in tests: a v5 dataset with digits 2/0/3 written
straight into IndexedDB migrates to a setting of 3, the table loses its
Decimals column, and editing the control persists schema 6 with digits stripped
from each currency and income left at 20000.55. The same month reads 600/206 at
zero decimals and 600.000/205.561 at three, so the figures recompute rather
than merely reformat.

Not verified here: the .ods in a real spreadsheet.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/domain/types.ts

@@ -9,11 +9,15 @@
  */
 export type Currency = string;
 
-/** What the owner has told the app about one currency. */
+/**
+ * What the owner has told the app about one currency.
+ *
+ * Decimal places are NOT here. They are one setting for the whole dataset,
+ * `Settings.digits`, applied to every currency alike — see
+ * docs/specs/2026-09-02-global-decimals-design.md, including what that costs.
+ */
 export interface CurrencyDef {
   code: Currency;
-  /** Decimal places in its minor unit: 2 for DKK, 0 for JPY, 3 for KWD. */
-  digits: number;
   /** "kr", "$", "€". Optional — a currency can be typed by code alone. */
   symbol?: string;
   /** "Danish krone". Shown in Settings; never used for identity. */
@@ -25,14 +29,14 @@ export interface CurrencyDef {
  * currencies — that lives in the dataset, because the owner can add more.
  */
 export const SEED_CURRENCIES: readonly CurrencyDef[] = [
-  { code: "DKK", digits: 2, symbol: "kr", name: "Danish krone" },
-  { code: "USD", digits: 2, symbol: "$", name: "US dollar" },
-  { code: "EUR", digits: 2, symbol: "\u20ac", name: "Euro" },
-  { code: "GBP", digits: 2, symbol: "\u00a3", name: "British pound" },
+  { code: "DKK", symbol: "kr", name: "Danish krone" },
+  { code: "USD", symbol: "$", name: "US dollar" },
+  { code: "EUR", symbol: "\u20ac", name: "Euro" },
+  { code: "GBP", symbol: "\u00a3", name: "British pound" },
 ];
 
-/** Used when a currency has no definition. Two places is right almost always. */
-export const DEFAULT_CURRENCY_DIGITS = 2;
+/** What a brand-new dataset rounds to. Right for every currency it ships with. */
+export const DEFAULT_DIGITS = 2;
 
 /** A float amount paired with its currency. See AGENTS.md for rounding rules. */
 export interface Money {
@@ -127,6 +131,14 @@ export interface Settings {
   baseCurrency: Currency;
   foldStartMonth: MonthId;
   schemaVersion: number;
+  /**
+   * Decimal places for EVERY amount, whatever its currency. Integer 0-4.
+   *
+   * Required, not optional: an optional field means a `?? 2` at every read,
+   * and a silent default of 2 is exactly the wrongness that the old
+   * per-currency table's required argument existed to prevent.
+   */
+  digits: number;
   /** Optional: no migration needed, existing datasets leave it undefined. */
   fxApiUrl?: string;
 }