Fix inline reader/highlight drifting to wrong article after refresh

[disnet]

Summary

• Track feed-list selection and expansion by FeedDisplayItem.key instead of by array index, so refresh-driven re-sorts no longer move the inline reader / highlight onto a different article.
• Keep selectedIndex / expandedIndex as derived getters so existing call sites that need an index (scroll-to-center, j/k navigation) keep working.

Why

If you click on an article in a channel and read it inline, then trigger (or auto-receive) a refresh that brings new articles in, the existing index-based selection stays put — but the array has been re-sorted by publishedAt, so the highlighted slot now belongs to a different article. The originally-clicked article appears un-expanded at its new index and is also marked-as-read because of the resolved selection. #204

Changes

• frontend/src/lib/stores/feedView.svelte.ts
  • Replace selectedIndex / expandedIndex state with selectedKey / expandedKey (string | null).
  • Add selectByKey(key); rewrite select(index) to resolve the index to a key via currentItems.
  • Expose selectedKey / expandedKey and keep selectedIndex / expandedIndex as derived getters (findIndex against currentItems).
• frontend/src/lib/components/feed/FeedListView.svelte
  • Selected/expanded/highlighted comparisons now compare against displayItem.key.
  • openSelectedReader resolves the item by key.
• frontend/src/lib/components/feed/SavedListView.svelte
  • Same key-based comparison swap for SavedCard.selected.
  • openSelectedReader resolves by key.
• frontend/src/lib/hooks/useFeedKeyboardShortcuts.svelte.ts
  • Add a getSelectedItem() helper that looks up the selected item by selectedKey.
  • Next/prev navigation computes the current index via findIndex(i => i.key === selectedKey) to preserve j/k behaviour.
  • hasSelected() now checks selectedKey !== null; the h condition checks expandedKey !== null.

Notes / risks

• readerItem already stored the item object directly; the fullscreen reader was never affected, only the list highlight and the inline preview.
• If a refresh removes the selected item entirely, selectedKey will not find a match in currentItems and the derived selectedIndex returns -1 — same observable state as before.
• FeedDisplayItem.key is already trusted as unique by the keyed #each in both list views; no new uniqueness assumption.

Test plan

• cd frontend && npm run check (passes, 0 errors)
• Manual: open a channel with N unread, click the 4th to expand inline, trigger a refresh that brings new items, confirm the originally-clicked article stays expanded at its new index and is the one marked as read.
• Manual: with an item selected, refresh the list, press j/k and confirm selection moves from the correct current article.
• Manual: open fullscreen reader on an article, refresh underneath, close reader → list focus returns to the right card.
• Manual: repeat (2) on the Saved channel via SavedListView.

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