fix(scheduler): harden scheduled operations with stale cleanup, cron validation, and design fixes (#549)

* fix(scheduler): clean up stale runs on startup and auto-disable invalid cron tasks

- Add markStaleRunsAsFailed() bulk DB method with status index
- Clean up orphaned 'running' records on scheduler startup
- Auto-disable tasks when cron expression becomes invalid at execution time
- Promote CRUD debug logs to standard logs for scheduled task admin actions
- Add diagnostic logging for task pre-checks, action timing, and prune fallback

* refactor(scheduling): extract shared types and fix design system violations

- Extract ScheduledTask, TaskRun, NodeOption to shared types file
- Extract getCronDescription and formatTimestamp to shared utilities
- Fix formatTimestamp falsy-zero null check
- Tighten last_status type to 'success' | 'failure' | null
- Add strokeWidth={1.5} to all action icons per design system
- Add sr-only DialogDescription for accessibility
- Wrap Sheet run history in ScrollArea
- Fix delete button styling to match design system pattern
- Change manual trigger toast from "executed" to "triggered"

* test(scheduler): add tests for snapshot, remote update, stale cleanup, and cron invalidation

- Add stale run cleanup tests (bulk markStaleRunsAsFailed, logging)
- Add cron invalidation test (auto-disable, error message)
- Add executeSnapshot tests (fleet capture, empty stacks)
- Add executeUpdateRemote tests (proxy success, remote error)
- Document stale run cleanup and cron auto-disable in troubleshooting docs

frontend/src/lib/scheduling.ts

@@ -0,0 +1,14 @@
+import cronstrue from 'cronstrue';
+
+export function getCronDescription(expression: string): string {
+  try {
+    return cronstrue.toString(expression);
+  } catch {
+    return 'Invalid expression';
+  }
+}
+
+export function formatTimestamp(ts: number | null): string {
+  if (ts == null) return '-';
+  return new Date(ts).toLocaleString();
+}