Improve workflow time selector and biweekly day toggle

[alari76]

Summary

• Narrowed the time input field and added color-scheme CSS so the native time picker dropdown and clock icon render correctly in both dark and light themes
• Replaced the standalone "Bi-weekly" preset button with a Weekly / Every 2 weeks toggle that appears when a specific day is selected, so users can choose both the trigger day and frequency
• Updated buildCron, parseCron, and describeCron to support biweekly-with-day encoding (e.g. biweekly-1 → 0 6 */14 * 1) with backward compatibility for legacy expressions

Test plan

• All 855 existing tests pass
• Verify time input is narrower and clock icon is visible in dark mode
• Verify native time picker dropdown has dark theme styling
• Select a day (e.g. Mon) and confirm Weekly/Bi-weekly toggle appears
• Toggle to "Every 2 weeks" and confirm cron description updates (e.g. "Bi-weekly Mon at 06:00")
• Switch days while in bi-weekly mode and confirm it stays bi-weekly
• Select Daily/Weekdays preset and confirm toggle hides
• Test in light mode — time picker should use light scheme

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