feat: refine month totals and recurring purchase entry

docs/ARCHITECTURE.md

@@ -62,6 +62,12 @@ aggregate envelope figures over `foldBalances` / `figuresFor`. Overall Spent
 also includes unassigned purchase slices, which have no envelope to fold. A
 closing balance is read from `byMonth[11].remaining`, never re-summed.
 
+The month header's Actual available is entered income minus all actual Spent
+for that month. It includes refunds, unassigned spending and finance-plan slices,
+but no allocation, carry or unpaid expectations. Before the rollover start,
+the month view reads the existing charge calculation directly because the fold
+has deliberately produced no envelope figures yet.
+
 A recurring cost adds a SECOND balance beside it, on its own carry:
 
 ```
@@ -157,6 +163,11 @@ still works.
 
 ## Money
 
+The default currency table seeds new datasets only. JPY, AUD, CAD and MXN join
+DKK, USD, EUR and GBP; existing currency tables are not rewritten. The build
+derives its FX targets from the seed table and carries complete committed
+fallback rates. These rates never replace one an owner has cleared.
+
 A float, deliberately, with two rules that make it safe:
 
 1. **Round at every boundary** via `roundMoney` — after any division, after any
@@ -187,6 +198,11 @@ which also says what re-adding a per-currency override would take.
 
 ## Splits and finance plans compose
 
+Purchase entry defaults to creating a new recurring cost when the owner chooses
+to make a purchase recur. Selecting an existing bill is enabled explicitly in
+Settings by the optional `allowExistingRecurringCost` boolean (absent is false),
+validated on JSON import. Existing purchase associations remain editable.
+
 A purchase can be split across posts *and* spread over months at once. The trick
 is in `charges.ts`: first compute how the **whole** purchase divides across posts
 (`splitPartsOfTotal`), then distribute each month's slice proportionally *to
@@ -211,6 +227,16 @@ explicit zero rule, including a zero monthly override, does have an overspend
 status when its balance is negative. The views use the presence of a rule rather
 than infer it from the numeric allocation, since both cases fold to zero.
 
+## Display preferences and build identity
+
+Monthly chart visibility and Expected's expanded state persist per browser,
+alongside the theme preference. They never enter an exported dataset or change
+the month calculations. Storage errors leave the display controls usable.
+
+Colophon receives a source ref and UTC build date separately from the build.
+The date describes the artifact, not the browser's current day. A source archive
+has no Git ref but still receives a unique service-worker cache version.
+
 ## Things the app deliberately does NOT validate
 
 Adding a "missing" guard here is a regression, not a fix: