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/store/migrations.ts

@@ -18,6 +18,13 @@ const V3_SEED_CURRENCIES = [
   { code: "EUR", digits: 2, symbol: "\u20ac", name: "Euro" },
 ];
 
+/**
+ * What 5 -> 6 falls back to when a dataset carries no usable per-currency
+ * digits. Frozen for the same reason as the tables above: a step's output must
+ * not move when today's defaults do.
+ */
+const V6_FALLBACK_DIGITS = 2;
+
 /** Sterling as 4 -> 5 adds it. Frozen for the same reason as the list above. */
 const V5_GBP = { code: "GBP", digits: 2, symbol: "\u00a3", name: "British pound" };
 const V5_GBP_RATE = {
@@ -135,6 +142,38 @@ const MIGRATIONS: Array<(data: any) => any> = [
       fxRates: hasGbpRate || hasGbp ? fxRates : [...fxRates, { ...V5_GBP_RATE }],
     };
   },
+
+  // 5 -> 6: decimal places stop being per-currency and become one setting for
+  // the whole dataset. See docs/specs/2026-09-02-global-decimals-design.md,
+  // which records what that gives up: a dataset mixing minor units — yen at 0
+  // beside kroner at 2 — can no longer be described.
+  //
+  // The setting takes the MAXIMUM of what the currencies declared, not the base
+  // currency's value. A dataset holding a three-decimal currency already has
+  // three-decimal amounts stored in it, and adopting the base's 2 would mean
+  // the next edit of one silently truncated it. The maximum can only ever keep
+  // more precision than before, never less.
+  //
+  // Behaviour-preserving on every dataset this app has written: all four seeded
+  // currencies record 2, so the maximum is 2 and not one stored amount rounds
+  // differently than it did before.
+  (data: any) => {
+    const currencies = Array.isArray(data.currencies) ? data.currencies : [];
+    const declared = currencies
+      .map((currency: any) => currency?.digits)
+      .filter((digits: unknown): digits is number => Number.isInteger(digits));
+    return {
+      ...data,
+      settings: {
+        ...data.settings,
+        schemaVersion: 6,
+        digits: declared.length > 0 ? Math.max(...declared) : V6_FALLBACK_DIGITS,
+      },
+      // `digits` dropped from each entry; everything else about a definition
+      // the owner wrote is theirs and survives untouched.
+      currencies: currencies.map(({ digits, ...rest }: any) => rest),
+    };
+  },
 ];
 
 export function migrate(raw: unknown): Dataset {