feat(lists): surface list contributors on the list detail header

[Adron]

Summary

iOS showed list watchers — people who have been granted access — but never contributors, the people who have actually written rows. On a shared list that is the more useful signal, and the web has surfaced it on the list header since ContributorAvatarStack landed. This adds the iOS equivalent: a tappable avatar stack in the ListDetailView header that opens a ranked sheet.

Closes #58

What's included

• API (Services/APIClient+ListContributors.swift, new): listContributors(listId:) hitting GET /api/lists/{id}/contributors. Lives in a per-feature extension file rather than APIClient.swift, per CLAUDE.md. The route returns the full ranked set with no server paging, so the client neither pages nor re-sorts — re-sorting would put iOS out of step with the web's order.
• Model (Models/ListContributor.swift, new): ListContributor (user + addedCount / editedCount / score) and a ListContributorsResult wrapper carrying totalContributors. Keys arrive camelCase off the backend's serialize(), which passes through the shared convertFromSnakeCase decoder unchanged.
• Views (Views/ListContributorsView.swift, new): ContributorAvatarStack — cascading avatars with a "+N" overflow bubble — and ListContributorsSheet, whose rows mirror the WatchersListView row style (36pt circular avatar, display name, @username · added N · edited M). Both have #Previews.
• Wiring (Views/ListsView.swift): the stack mounts as a tappable header section in ListDetailView, fetched concurrently with the existing schema/items/connections loads rather than adding a serial round-trip to every list open.
• Project: all four new .swift files registered in project.pbxproj via the xcodeproj gem (no synced groups in this project).

Behavior gates

• GitHub-backed lists: the affordance is hidden and the request is not even issued — the route returns an empty set for them by design.
• Below two contributors: nothing renders, matching the web's totalContributors < 2 threshold, so a single-owner list stays uncluttered.
• Failure is non-fatal: a failed or 403'd contributor fetch leaves the stack hidden rather than failing the whole list load.

Not changed

No row-write path was touched. GitHub row writes still go through the full-row updateItem — a partial PUT would rebuild the issue and rename it to "Untitled". The diff on ListsView.swift is purely additive; updateItem, setGitHubState, saveEdit, and updateField are untouched.

Testing

• xcodebuild -scheme InterlinedList -destination 'platform=iOS Simulator,id=<pinned UDID>' -parallel-testing-enabled NO build — BUILD SUCCEEDED
• Full unit suite (E2E skipped, serialized) — 954 tests, 0 failures
• New APIClientListContributorsTests — 12 tests, 0 failures, covering: route + method, list-id percent-encoding, decode of the aggregation counts, preservation of server ranking, null and empty displayName fallback, the web-matching added N · edited M subtitle, the empty set GitHub-backed lists return, 401/403 propagation, and the stack's accessibility-label pluralization.

Accessibility

.accessibilityLabel on the stack reads "3 contributors" (singularizing at one), with a hint that it opens the full list; sheet rows announce name plus added/edited counts.

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