fix(notifications): handle a missing X-Total-Count and serialise unread fetches

[tagpro]

Follow-up to #2, which is already on master. This carries only the review fixes from upstream maxdorninger/MediaManager#564 — one file, no re-application of the pagination work.

Missing X-Total-Count collapsed the totals to zero

Number(null) is 0, not NaN, so the Number.isNaN fallback in totalCountOf never ran. If the header were absent, unreadTotal/readTotal would be set to 0 while notifications were still rendered — hiding the unread count and the "Mark All as Read" button. It now falls back on a null or empty header and only accepts a finite number, so a genuine "0" still reads as zero.

The 30s poll could race with "Load More"

Both write unreadNotifications, and they could overlap: a poll issued before a "Load More" but resolving after it would overwrite the list with page one, discarding the page the user just asked for (or repeating an id in the keyed {#each}). Unread fetches now go through a promise queue, so each request reads its offset only once the previous one has settled — this covers the reverse direction too (a "Load More" clicked while a poll is in flight). The interval additionally skips while loadingMoreUnread is set, since a queued reset would otherwise truncate the list back to the first page right after a "Load More" resolved.

Verification

• Simulated the overlap: with the old concurrent code the list ended up jumping straight from row 49 to row 100 — an entire page silently dropped from the middle. With the fetches queued it stays contiguous.
• Checked the header coercion across null, "", "0", "12", "abc": null/"" now fall back instead of yielding 0, and "0" still yields 0.
• npm run lint (in web/) — clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01SC2BDd7wA7hPa7k2h2yAv1

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Reviewing files that changed from the base of the PR and between e070dc6 and d1b12ff.

📒 Files selected for processing (1)

• web/src/routes/dashboard/notifications/+page.svelte

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• Commit unit tests in branch fix/notifications-load-race

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