## Library/Gallery Modal Improvements
### Performance
- Library items are cached so reopening the modal is instant instead of
loading from scratch every time
- Only visible sections are rendered - off-screen groups are swapped out
as you scroll, keeping things smooth even with lots of media
- Username session is cached to skip a redundant API call on every open
### UX
- Replaced loading spinner with a skeleton grid placeholder
- New items from completed generations appear at the top without wiping
the library
- Library auto-refreshes when a generation finishes (if open)
- Refresh icon spins while refreshing
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