v0.1.5: detect free windows in the estimate fallback + add API tests

App:
- When the single-estimate fallback sees MaxTime 0 (or an all-$0.00 ladder),
  show "Parking is currently free" instead of a bogus $0.00 purchase ladder.
  Free-detection extracted to tested client helpers (isFreeEstimate/ladderAllFree).

Client + tests (node:test, zero deps):
- src/estimates.ts: pure isFreeEstimate/ladderAllFree helpers (exported).
- test/estimates + test/http (mocked fetch): headers, query serialization,
  error->ParkSmarterApiError, rolling Auth_Token refresh, 401 clears token.
- test/integration: live checks against the Sandpoint (DSB) zones; targets the
  `test` instance by default and SKIPS when it's unreachable (non-prod is
  IP-restricted -> 403). Verified 3/3 green against prodv2 read-only.
- CI runs `npm test` as a build gate.

Note: dev/stage/test instances return 403 "Web App - Unavailable" from the
public internet (IP-restricted). API schema is otherwise unchanged vs Jul 6.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

parksmarter-client/src/estimates.ts

@@ -0,0 +1,24 @@
+/**
+ * Helpers for interpreting ParkSmarter parking estimates.
+ *
+ * Pulled out as pure functions so the "is this a free window?" decision is
+ * testable without the app, the network, or React Native. The BigBrainParking
+ * app's single-estimate fallback uses these to avoid showing a bogus "$0.00"
+ * purchase ladder during free windows.
+ */
+
+/**
+ * A free window: the single estimate reports no purchasable time. Live captures
+ * show free zones return `MaxTime: 0` (and `$0.00`), while paid zones return a
+ * positive MaxTime. Anything <= 0 (or non-numeric) counts as free.
+ */
+export function isFreeEstimate(probe: { MaxTime?: number | string | null } | null | undefined): boolean {
+  return (Number(probe?.MaxTime) || 0) <= 0;
+}
+
+/** Every rung of a built ladder is $0.00 — effectively a free window too. */
+export function ladderAllFree(
+  rungs: ReadonlyArray<{ ParkingCost?: number | string | null }>,
+): boolean {
+  return rungs.length > 0 && rungs.every((r) => (Number(r?.ParkingCost) || 0) === 0);
+}