feat(webapp): default the cancel in-progress runs option to on for billing limits

**Before:** the first time you configured a billing limit, the "Cancel
in-progress runs when this limit is reached" checkbox started unchecked.
Unless you noticed it and ticked it, runs that were already executing
when the limit was reached kept going until they finished, so spend
could keep climbing past the limit even though new triggers were already
being held.

**After:** the checkbox now starts checked for a limit that has not been
configured yet, so cancelling already-executing runs is the default.
Organizations that have already saved a billing limit keep whatever they
chose, including an explicit opt-out.

**How:** the form's initial checked state for an unconfigured limit is
now on instead of off. Nothing about saved settings or submitted values
changed: a missing checkbox on submit is still read as off, so unticking
the box and saving still disables cancellation.
Mono-RevId: c1068f09f9a8db53e45bf3f91c879569becccb25

Author: claude[bot]

.server-changes/billing-limit-cancel-in-progress-default-on.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+The "Cancel in-progress runs when this limit is reached" option on the billing limit form is now enabled by default when you first configure a limit, so already-executing runs stop instead of continuing past the limit. Organizations that have already saved a billing limit keep their existing choice.

apps/webapp/app/components/billing/BillingLimitConfigSection.tsx

@@ -115,7 +115,10 @@ export function BillingLimitConfigSection({
     billingLimit.isConfigured && billingLimit.mode === "custom"
       ? (billingLimit.amountCents / 100).toFixed(2)
       : "";
-  const savedCancelInProgressRuns = billingLimit.isConfigured && billingLimit.cancelInProgressRuns;
+  // Unconfigured limits default to cancelling in-progress runs; saved configs keep their stored choice.
+  const savedCancelInProgressRuns = billingLimit.isConfigured
+    ? billingLimit.cancelInProgressRuns
+    : true;
 
   // Unconfigured limit starts with nothing selected.
   const resetMode: "" | "none" | "plan" | "custom" = billingLimit.isConfigured ? savedMode : "";