feat(creators): add infinite scroll to the creator directory

[Deb-Auth]

Summary

Adds the creator directory at /creators and loads its creators lazily, a page at a time, instead of rendering the whole index up front.

closes #391

Note: frontend/app/creators/ did not exist on main, so this PR introduces the directory page itself along with the lazy loading the issue asks for. Everything is scoped to the new route; no existing file is modified.

Lazy loading

app/creators/hooks/useInfiniteScroll.ts

A small hook around the Intersection Observer API:

• Returns a ref callback rather than a ref object, so the observer re-attaches when the sentinel is mounted, unmounted or replaced. This matters because the sentinel only exists while there are further pages.
• enabled gates observation: it is false while a page is in flight, once the last page has loaded, and after an error, so the observer cannot fire redundant or looping loads.
• rootMargin defaults to 200px, so the next page is typically in place before the user reaches the end of the list.
• The onLoadMore callback is held in a ref, so a new callback identity does not tear down and rebuild the observer.
• Environments without IntersectionObserver are handled by simply never triggering, which is why the page also keeps a manual control (below).

app/creators/services/creatorService.ts

The backend has no dedicated creators endpoint, so the directory is derived from the public global certificate index that already powers /search:

• fetchCreators({ page, pageSize, search, signal }) requests one page of certificates and returns { creators, hasMore }, where hasMore is true when the index returned a full page.
• groupCertificatesByCreator aggregates a page into per-creator entries (asset count, most recent mint, asset categories), newest activity first.
• Because grouping happens per page, a creator can legitimately reappear in a later page. mergeCreators folds repeats into the already-loaded entry rather than pushing a duplicate card, and preserves the existing order so the grid never reshuffles under the user mid-scroll.

app/creators/page.tsx

• Renders the grid of CreatorCards plus a sentinel below it; scrolling the sentinel into view appends the next page.
• Debounced search input (300 ms) that restarts pagination from the first page.
• Loading skeletons for the first page, an inline "Loading more creators" indicator for subsequent pages, an error state with a retry action, and an empty state.
• A "Load more" button rendered alongside the sentinel, so keyboard users and browsers without Intersection Observer can page through the directory too.
• Requests are aborted through an AbortSignal on unmount and whenever the query or page changes, so a stale response can never overwrite a newer one.
• Loading flags are raised by whatever triggers a load (mount, new query, load more, retry) rather than inside the effect, keeping the effect body free of synchronous setState calls.

app/creators/components/CreatorCard.tsx renders one creator: monogram, display name with truncated wallet address, verified asset count, last mint date and category chips, linking through to that creator's assets in search. Address and date formatting reuse the existing app/search/components/shared helpers.

Verification

• pnpm --filter web run build compiles successfully and /creators is emitted as a route.
• pnpm --filter web run lint reports no errors or warnings from the new files.

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