feat: wallet operations engine and encrypted storage with tests

src/lnurlcash/units.ts

@@ -0,0 +1,21 @@
+// The protocol layer speaks msat everywhere (LUD-25 amounts, kit fee math);
+// humans think in sats. These helpers are the only place the two meet.
+
+export const MSAT_PER_SAT = 1000
+
+export const msatToSats = (msat: number): number => msat / MSAT_PER_SAT
+
+export const satsToMsat = (sats: number): number =>
+  Math.round(sats * MSAT_PER_SAT)
+
+// rounds up to the next whole sat - for an msat amount about to be
+// requested as an invoice, where sub-sat precision (e.g. from a mint fee's
+// percentage cut, see grossUpForMintFee) isn't reliably payable
+export const ceilMsatToSat = (msat: number): number =>
+  Math.ceil(msat / MSAT_PER_SAT) * MSAT_PER_SAT
+
+// rounds down to the nearest whole sat - for a fee-adjusted amount shown
+// as an upper bound: rounding up there would advertise a note value that
+// isn't actually reachable
+export const floorMsatToSat = (msat: number): number =>
+  Math.floor(msat / MSAT_PER_SAT) * MSAT_PER_SAT