fix(updates): isolate feed failures so one feed can't break the page

onMount fetched the novel feed then the manga feed, each via
`(await fetch(...)).json()` with no error handling. When a feed endpoint
failed (e.g. all-novels, when wlnupdates returns HTML), `.json()` threw
and aborted onMount before the other feed loaded, leaving the whole page
stuck on skeletons. Load each feed through a fetchJson helper that
returns null on any failure (non-ok, network, or parse error) and set
the feed to null — the existing "Failed to load feed" state — when the
response is missing or malformed. A failing feed now degrades gracefully
while the other still renders.

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