Add fixed daily playlist refresh scheduling

[jrnwndr]

Summary

Adds an optional fixed daily refresh time to playlists.

Motivation

A 24-hour playlist is useful for a dashboard that should update once per
day. With a duration-based refresh interval, the refresh can drift when a
device is manually fetched or reconnects at another time. A fixed daily
refresh keeps the playlist aligned with a predictable local time, such as
midnight.

Changes

• Add an optional dailyRefreshAt playlist field in HH:MM format
• Validate and persist the configured refresh time
• Calculate the next refresh in the configured DEFAULT_TIMEZONE
• Handle daylight-saving time transitions
• Give the daily schedule precedence over per-screen durations
• Disable duration fields in the UI while daily refresh is enabled
• Document the feature in the README
• Add coverage for regular days and DST transitions

Verification

• Backend tests passing
• Frontend tests passing
• Frontend production build passing
• Manually verified with an e-ink device using the display API