fix(frontend): render exact batches in relay list and reset paging on new matches

[JuliusHenke]

Follow-up to #249 (resolved by d89dd7b in #569, which replaced react-infinite-scroller with an IntersectionObserver sentinel). Two small issues remained in RelayList; both existed before the library switch:

• Off-by-one: each batch rendered 51 relays instead of 50 (slice(0, numberOfMatchesToDisplay + 1)).
• No reset on new matches: the number of rendered relays was kept when relayMatches changed (e.g. after a search), so a new list rendered every previously loaded batch at once. The list now starts again with the first batch. It uses the same "synced state during render" pattern as ResponsiveRelayDetailsDialog and RelayDetailsDialogSmall. relayMatches is memoized upstream, so the reset only fires on a real change.

Tests

• New RelayList.test.tsx (browser mode): the first render shows exactly 50 relays plus the loader; after loading all 120 and swapping in a new list, only 50 are shown again.
• Both new tests fail on master (51 rows shown; 120 rows shown after the swap) and pass with this change.
• yarn test:run (179 tests in 15 files), yarn lint and yarn build pass locally.

No dependency changes, so no SCA run or dependency vetting was needed.

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https://claude.ai/code/session_012ddTD9i5zX7dGHoYf8sZSS

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