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.

docs/PRODUCT.md

@@ -74,11 +74,17 @@ Confirmed and in place:
 - Multi-currency (DKK/USD/EUR) against a user-maintained exchange-rate table,
   with an optional on-demand fetch that fails soft and is never required.
 - JSON export/import and ODS (spreadsheet) export, both generated in the
-  browser.
+  browser. On each app start, a separate browser-stored session backup is made
+  before editing is available; recent daily, weekly and monthly checkpoints can
+  be downloaded or restored from Settings. These checkpoints share the browser
+  origin with the live data, so JSON export is still needed for protection
+  against losing browser storage.
 - Offline-capable installable PWA. Currently deploys as one static artifact to
   both GitHub Pages (under a subpath) and a self-hosted Docker image.
 - Recurring costs — a bill that repeats every N months, every N days, or
-  every N weeks on a weekday, split across posts like any purchase. A cost is
+  every N weeks on a weekday, optionally split across posts like any purchase.
+  A cost may have no post: it remains expected without moving an envelope's
+  projected balance, and confirming it records unassigned spending. A cost is
   anchored one of two ways: `calendar` holds a fixed grid of due dates, so
   paying two months at once still leaves nothing due until the one after; a
   cost anchored to its **last charge** has no grid at all and can be charged