feat: refine month totals and recurring purchase entry

docs/PRODUCT.md

@@ -71,6 +71,9 @@ Confirmed and in place:
 - A purchase may have no post. It remains visible in its month and counts in
   Spent, while leaving envelope balances unchanged.
 - Per-month income, entered directly, varying month to month.
+- The month header shows actual available: that month's entered income minus
+  actual spending, including unassigned purchases and refunds. Monthly breakdown
+  can be hidden and Expected collapsed; these choices persist in this browser.
 - 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
@@ -92,8 +95,9 @@ Confirmed and in place:
   whichever charge was most recent — a phone plan that recharges early when
   its data runs out is the case this exists for. Each due date on the
   calendar model is a projected occurrence the owner confirms into a real
-  purchase; a recurring cost can also be tagged onto a purchase, or created
-  from one, while it is being entered, rather than only from Settings. The
+  purchase; a recurring cost can be created from a purchase while it is being
+  entered. Choosing an existing bill in Add purchase is enabled explicitly in
+  Settings; existing purchase associations remain editable. The
   month view shows what is still expected alongside a second, projected
   balance per post, so a known bill is visible before it is paid without
   touching the reconciled figure a bank statement is checked against.