feat: add month breakdown and session backups

Allow recurring costs and confirmations to remain unassigned when created from a purchase without a post. Add browser backup retention and restore controls, with coverage and documentation updates.

src/ui/monthPie.ts

@@ -0,0 +1,29 @@
+import type { MonthViewModel } from "../domain/views.ts";
+
+export type MonthPieMode = "spending" | "allocation";
+export interface MonthPieSlice {
+  id: string;
+  label: string;
+  amount: number;
+}
+
+/** Chart data comes from the same folded figures as the month table. */
+export function monthPieData(view: MonthViewModel, mode: MonthPieMode) {
+  const items: MonthPieSlice[] = view.rows.map(({ post, figures }) => ({
+    id: post.id,
+    label: post.name,
+    amount: mode === "spending" ? figures.charges : figures.allocation,
+  }));
+  if (mode === "spending" && view.looseCharges !== 0) {
+    items.push({ id: "unassigned", label: "Unassigned", amount: view.looseCharges });
+  }
+  const byAmount = (a: MonthPieSlice, b: MonthPieSlice) => b.amount - a.amount || a.label.localeCompare(b.label);
+  const positive = items.filter((item) => item.amount > 0).sort(byAmount);
+  const adjustments = items.filter((item) => item.amount < 0).sort((a, b) => a.amount - b.amount);
+  return {
+    positive,
+    adjustments,
+    positiveTotal: positive.reduce((sum, item) => sum + item.amount, 0),
+    netTotal: mode === "spending" ? view.totalCharges : view.totalAllocation,
+  };
+}