feat(ui): add month view with income entry and post table

Negative remaining balances are shown in a warning treatment; nothing
blocks going over budget.

Applies the Tasks 18-23 design contract: --paper/--ink/--rule/--accent/
--surplus/--overspend tokens and the three type-role font variables in
src/index.css, --font-money on every money figure, and the carry meter
(a 3px hard-stopped gradient beneath each PostTable row) encoding
carried-in surplus, allocation track, spend fill and overrun.

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

src/ui/format.ts

@@ -0,0 +1,20 @@
+import type { Currency } from "../domain/types.ts";
+
+// Pinned locale: output must not vary between the user's machine and CI.
+const NUMBER = new Intl.NumberFormat("en-GB", {
+  minimumFractionDigits: 2,
+  maximumFractionDigits: 2,
+});
+
+export function formatAmount(amount: number): string {
+  return NUMBER.format(amount);
+}
+
+export function formatMoney(amount: number, currency: Currency): string {
+  return `${formatAmount(amount)} ${currency}`;
+}
+
+export function formatSignedMoney(amount: number, currency: Currency): string {
+  const sign = amount > 0 ? "+" : "";
+  return `${sign}${formatMoney(amount, currency)}`;
+}