feat(scheduled-tasks): add cron syntax support to task editor dialog (#1593)

Adds a 'Cron' schedule type option to the scheduled task editor, allowing
users to create and edit cron-based schedules through the UI.

- Add cron expression input with inline validation (cron-parser)
- Show next 4 upcoming run times as a preview
- Provide clickable example chips (every 5min, hourly, Monday 9am, etc.)
- Preserve cron expressions when editing existing cron tasks
- Add cron.ts utility module for validation and next-run computation
- Add i18n keys across all 8 locale files

Closes #1586

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/lib/cron.ts

@@ -0,0 +1,51 @@
+import { CronExpressionParser } from 'cron-parser';
+
+export type CronValidationResult = {
+  valid: boolean;
+  error?: string;
+};
+
+export function isValidCronExpression(expression: string): CronValidationResult {
+  if (!expression || !expression.trim()) {
+    return { valid: false, error: 'required' };
+  }
+  try {
+    CronExpressionParser.parse(expression);
+    return { valid: true };
+  } catch (err) {
+    const message = err instanceof Error ? err.message : 'Invalid cron expression';
+    return { valid: false, error: message };
+  }
+}
+
+export function getNextRuns(
+  expression: string,
+  timezone: string,
+  count: number = 4,
+): Date[] {
+  try {
+    const options = timezone ? { tz: timezone, currentDate: new Date() } : { currentDate: new Date() };
+    const interval = CronExpressionParser.parse(expression, options);
+    const dates: Date[] = [];
+    for (let i = 0; i < count; i += 1) {
+      const next = interval.next();
+      if (next) {
+        dates.push(next.toDate());
+      }
+    }
+    return dates;
+  } catch {
+    return [];
+  }
+}
+
+export const CRON_EXAMPLES: ReadonlyArray<{
+  expression: string;
+  labelKey: string;
+}> = [
+  { expression: '*/5 * * * *', labelKey: 'sessions.scheduledTasks.editor.cronExpression.examples.every5min' },
+  { expression: '0 * * * *', labelKey: 'sessions.scheduledTasks.editor.cronExpression.examples.everyHour' },
+  { expression: '0 9 * * 1', labelKey: 'sessions.scheduledTasks.editor.cronExpression.examples.monday9am' },
+  { expression: '0 9,17 * * *', labelKey: 'sessions.scheduledTasks.editor.cronExpression.examples.9am5pm' },
+  { expression: '0 0 1 * *', labelKey: 'sessions.scheduledTasks.editor.cronExpression.examples.firstOfMonth' },
+];