feat(accounts): merchants and payers as a league table

Expense and revenue accounts list this month per currency, share,
visits, average, twelve months, typical and the change against it, last
visit and category; quiet ones fold into one line behind i. 1 to 5 sort,
; groups by category with subtotals, x and s c set the category on all
of a merchant's transactions in the period.

src/lib/accounts/merchants.svelte.ts

@@ -0,0 +1,89 @@
+import { createQuery, keepPreviousData } from "@tanstack/svelte-query";
+import { lists } from "#lib/api/lists.js";
+import type { TransactionGroup } from "#lib/api/types.js";
+import { ratesQuery } from "#lib/dashboard/primary.svelte.js";
+import { today } from "#lib/dates.js";
+import { period } from "#lib/period.svelte.js";
+import { ledgerQuery, ledgerWindow, ownFlows } from "#lib/reports/ledger.js";
+import { pages } from "#lib/reports/queries.js";
+import { typicalMonths } from "#lib/reports/typical.js";
+import { session } from "#lib/session.svelte.js";
+import { board, type Kind, monthDay } from "./merchants.js";
+
+export function useMerchants(kind: () => Kind) {
+  const now = today();
+  const current = ledgerWindow(now);
+  const anchor = $derived(period.end >= current.start ? now : period.end);
+  const window = $derived(ledgerWindow(anchor));
+  const inside = $derived(
+    period.start >= window.start && period.end <= window.end
+  );
+
+  const accounts = createQuery(() => lists.accounts("all"));
+  const ledger = createQuery(() => ({
+    ...ledgerQuery(anchor),
+    placeholderData: keepPreviousData,
+  }));
+  const extra = createQuery(() => ({
+    enabled: !inside,
+    placeholderData: keepPreviousData,
+    queryFn: () =>
+      pages<TransactionGroup>("/v1/transactions", {
+        end: period.end,
+        start: period.start,
+        type: "default",
+      }),
+    queryKey: ["merchants-period", period.start, period.end],
+  }));
+  const rates = createQuery(() => ({ ...ratesQuery(), enabled: true }));
+
+  const flows = $derived(
+    ledger.data && accounts.data ? ownFlows(ledger.data, accounts.data) : []
+  );
+  const periodFlows = $derived.by(() => {
+    if (inside) {
+      return flows;
+    }
+    return extra.data && accounts.data
+      ? ownFlows(extra.data, accounts.data)
+      : [];
+  });
+  const day = $derived(monthDay(period.range, now));
+  const result = $derived.by(() => {
+    if (!(accounts.data && ledger.data && (inside || extra.data))) {
+      return null;
+    }
+    const month = period.end.slice(0, 7);
+    const since = flows.reduce(
+      (first, flow) => (flow.date < first ? flow.date : first),
+      now
+    );
+    const primary = session.primary?.code ?? "";
+    return board(accounts.data, flows, periodFlows, {
+      day,
+      kind: kind(),
+      months: window.months.filter((m) => m <= month),
+      past: typicalMonths(window.months, period.end, since),
+      range: period.range,
+      rate: (code) => rates.data?.[code]?.rate ?? (code === primary ? 1 : 0),
+    });
+  });
+
+  return {
+    get board() {
+      return result;
+    },
+    get day() {
+      return day;
+    },
+    get error() {
+      return accounts.error ?? ledger.error ?? extra.error ?? null;
+    },
+    get loading() {
+      return result === null || rates.isPending;
+    },
+    get stale() {
+      return ledger.isFetching || extra.isPlaceholderData;
+    },
+  };
+}