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>

docs/DECISIONS.md

@@ -18,7 +18,7 @@ record.
   no household mode. → [`PRODUCT.md`](PRODUCT.md)
 - **Schema migrations are mandatory, not optional.** `schemaVersion` plus
   ordered migration functions applied on load, because IndexedDB holds the
-  user's only copy of their financial data. Now at version 5.
+  user's only copy of their financial data. Now at version 6.
   → [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
 - **A destructive write downloads a backup first.** Import and reset both export
   before replacing, and abort if the export fails. → commit `ac3d18a`
@@ -33,13 +33,21 @@ record.
   last part as `total - sum(others)`.
   → [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
 - **`roundMoney` takes a digit count, and the argument is required.** Rejected a
-  default of 2: it is correct for every currency the app shipped with, so a
-  missed call site would pass every test and fail on the first zero-decimal
-  currency. → [`AGENTS.md`](../AGENTS.md) §1, commit `cdece63`
-- **`Currency` is an open string code; `Dataset.currencies` is the authority.**
-  Rejected a closed union, which would have made "add a currency" a code change.
-  The compile-time safety is replaced by validation at the boundaries —
-  `parseDatasetJson` rejects a code no currency defines.
+  default of 2: it is correct while the decimals setting is 2, so a missed call
+  site would pass every test and fail the moment it moves.
+  → [`AGENTS.md`](../AGENTS.md) §1, commit `cdece63`
+- **Decimal places are ONE dataset-wide setting, not per currency.**
+  `Settings.digits`; `CurrencyDef.digits` is gone. Chosen over a per-currency
+  default and over display-only rounding, and knowingly over correctness for
+  mixed minor units: a dataset holding both yen and kroner can no longer be
+  described, and the app does not warn. The owner asked for the simpler model
+  with that trade-off in front of them.
+  → [`specs/2026-09-02-global-decimals-design.md`](specs/2026-09-02-global-decimals-design.md)
+- **`Currency` is an open string code; `Dataset.currencies` is the authority**
+  on which codes exist, their names and their symbols — but no longer on their
+  decimals. Rejected a closed union, which would have made "add a currency" a
+  code change. The compile-time safety is replaced by validation at the
+  boundaries — `parseDatasetJson` rejects a code no currency defines.
   → `src/domain/types.ts`, commit `cdece63`
 - **Every derived number comes from one left fold.** `src/domain/fold.ts` is the
   only implementation; year and summary views aggregate over it rather than