fix(webapp): skip cron back-calc for inactive schedules in list

ScheduleListPresenter was deriving "Last run" via cron's previous slot
even for deactivated schedules, causing the dashboard to show "1 minute
ago" for a schedule that was actually deactivated months ago.

Gate the cron back-calc on `schedule.active`. Inactive schedules now
show "–" in the Last run cell, matching the semantic that they aren't
firing.

Per Devin review on PR #3476.

Author: ericallam

apps/webapp/app/presenters/v3/ScheduleListPresenter.server.ts

@@ -246,21 +246,25 @@ export class ScheduleListPresenter extends BasePresenter {
     });
 
     const schedules: ScheduleListItem[] = rawSchedules.map((schedule) => {
-      // Approximate "last run" from the cron's previous slot. If that slot
-      // predates the schedule itself, the schedule hasn't fired yet — show
-      // undefined rather than a misleading timestamp. UI is best-effort;
-      // accurate run history is on the schedule's runs page. cron-parser
+      // Approximate "last run" from the cron's previous slot. Skip inactive
+      // schedules — the cron's previous slot reflects what *would* have
+      // fired, but a deactivated schedule didn't actually fire there. Skip
+      // schedules whose cron's previous slot predates their creation — the
+      // schedule hasn't existed long enough to have fired. cron-parser
       // throws on malformed expressions, so degrade to undefined per-row
-      // rather than failing the whole list.
+      // rather than failing the whole list. UI is best-effort; the runs
+      // page is the source of truth.
       let lastRun: Date | undefined;
-      try {
-        const cronPrev = previousScheduledTimestamp(
-          schedule.generatorExpression,
-          schedule.timezone
-        );
-        lastRun = cronPrev.getTime() > schedule.createdAt.getTime() ? cronPrev : undefined;
-      } catch {
-        lastRun = undefined;
+      if (schedule.active) {
+        try {
+          const cronPrev = previousScheduledTimestamp(
+            schedule.generatorExpression,
+            schedule.timezone
+          );
+          lastRun = cronPrev.getTime() > schedule.createdAt.getTime() ? cronPrev : undefined;
+        } catch {
+          lastRun = undefined;
+        }
       }
 
       return {