Fix feed pagination offset for mixed items

[raymondjacobson]

Summary

• advance Latest feed pagination by the number of mixed feed items returned, not only tracks
• align the client offset with the API contract for /v1/users/{id}/feed, which applies OFFSET over tracks + collections
• prevents overlapping/repeated pages when collections are present in the feed

Context

Past commits introduced mixed feed rendering and then changed Latest feed pagination to count only track entries. That makes the client request page 2 with an offset that is too small whenever page 1 includes collections. Example: 6 tracks + 4 collections with limit 10 requests offset 6 instead of 10, so the next API page overlaps already-rendered items and can make pagination feel stuck.

The API history also shows separate server-side feed perf fixes (#798 for Latest feed planner cliffs, #805/#806/#817 for For You timeouts). This PR addresses the current client-side mixed-item offset mismatch.

Testing

• git diff --check

Could not run TypeScript locally because this worktree has no node_modules and the shell has node but no npm/npx installed.

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