docs+chore: everyNMonths is n-generic too; stop the demo double-charging Rent

PRODUCT.md described recurring costs as repeating "monthly, every N days,
or every N weeks", understating everyNMonths, which supports an interval
like the other two kinds.

demo-data.ts gave "Rent" both an ordinary monthly purchase and a
never-confirmed recurring cost from 2026-06 onward, so the demo's Rent
Projected read as double-charged for June through September. The ordinary
purchase now stops once the recurring cost exists.

docs/PRODUCT.md

@@ -67,8 +67,8 @@ Confirmed and in place:
   browser.
 - 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 monthly, every N days, or every N
-  weeks on a weekday, split across posts like any purchase. Each due date is a
+- 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. Each due date is a
   projected occurrence the owner confirms into a real purchase; 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

scripts/demo-data.ts

@@ -104,7 +104,12 @@ const buy = (date: string, description: string, amount: number, currency: "DKK"
 
 for (let m = 1; m <= 9; m++) {
   const mm = String(m).padStart(2, "0");
-  buy(`2026-${mm}-01`, "Rent", 8200, "DKK", rent.id);
+  // The "Rent" recurring cost above starts 2026-06 and is left unconfirmed,
+  // so it already projects an expected charge for June onward. An ordinary
+  // Rent purchase in those same months would double-charge the post in the
+  // screenshots — one from `charges`, one from `expected` — so this ordinary
+  // purchase only covers the months before the recurring cost exists.
+  if (m < 6) buy(`2026-${mm}-01`, "Rent", 8200, "DKK", rent.id);
   buy(`2026-${mm}-04`, "Groceries", 1180 + m * 37, "DKK", food.id);
   buy(`2026-${mm}-18`, "Groceries", 1420 - m * 12, "DKK", food.id);
   buy(`2026-${mm}-06`, "Commuter pass", 1100, "DKK", transport.id);