Paginate saved items list with infinite scroll

[disnet]

Summary

Saved-item views (inbox, archive, and saved channels) now render in 50-item pages and load more via the existing InfiniteScrollSentinel, instead of dumping the whole merged list into the DOM at once. Fixes the slowdown the user reported on large saved lists.

The sentinel is already mounted in SavedListView.svelte; only feedView.svelte.ts needed work.

• Extract savedItemsAll: the full merged/sorted/filtered saved list, pre-pagination.
• currentItems now slices to loadedArticleCount in saved view.
• hasMore returns loadedArticleCount < savedItemsAll.length for saved view (was hardcoded false).
• loadMore checks isSavedView before the viewMode dispatch — saved channels still report viewMode === 'articles', so the saved-specific branch must run first or it would page filteredArticles.length (article-only) instead of the merged saved-item count.
• setSavedView resets the page cursor on inbox ↔ archive toggle. Other entry points (setFilters, loadArticles) already reset.

Page size is DEFAULT_PAGE_SIZE = 50, matching articles/combined modes — single knob.

Test plan

• npm run check from frontend/ → 0 errors (pre-existing 25 a11y warnings only, unrelated files).
• With >100 saved items, navigate to /saved → only first 50 cards render initially.
• Scroll to bottom → next 50 append, no flicker, scroll position preserved.
• Continue paging until end → sentinel stops, total matches the full saved-items count.
• Toggle inbox → archive → inbox: cursor resets, paging still works.
• Apply a date / reading length / domain / tag filter on a saved channel and a sort change → list updates and paging works against the filtered total.
• Open a reader on item ~49, close → scroll position restored.

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