feat: show current EPG program in channel list with low-priority rendering

- Display current program title next to group name in each channel list
  item, separated by a dot separator
- Compute current program map in ChannelList using useMemo with 60s
  timer refresh via useTransition for non-blocking updates
- Use useDeferredValue for epgData to prevent large EPG loads from
  blocking user interactions (search, channel clicks)
- Wrap setEpgData and setIsMobile in startTransition for low-priority
  rendering of expensive state updates

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: stackia