gateway ui: page the run call log instead of truncating at 50

[fayekelmith]

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[Evanfeenstra]

Tested locally (PR UI against a mock run with 137 calls): paging works — 1–50 / 51–100 / 101–137, no gaps or dupes, Prev/Next disable correctly, page 1 shares the header query. 👍

A few things:

1. "First seen" is wrong for runs > 50 calls. It reads the oldest row of the first page, so on the 137-call run it showed call lsp-test-force-no-node-modules #88's time instead of the run start. Pre-existing, but this PR is exactly the case where it shows up.
2. Table flashes "Loading…" on every page change (page collapses then re-expands). placeholderData: keepPreviousData in useRunDetail would fix it.
3. Nit: consider paging by pagination.total_count (currently dropped by the gateway) instead of stats.total_requests — I think Bifrost's stats may exclude in-flight calls, though I didn't verify.