feat(question-feed): credit each illustration cluster via ArtistSupportLink

[WilfordGrimley]

Summary

Adds an artist credit to each Level 2 illustration cluster in the "What's
That Card?" question feed, reusing the existing ArtistSupportLink MTGAC
applet rather than plain text - the same applet already used for the
artist question type's post-answer moment and the /display rail's
Artist section.

Each IllustrationGroup (candidates sharing a Scryfall illustrationId)
now renders, once, above its CandidateGrid:

• Illustration by <Name> (plain text)
• the ArtistSupportLink applet for that name (MTGAC page link + any
  commerce links), wrapped in a caller-side max-width container so one
  full-bleed CTA per cluster doesn't read as page-width inside a grid that
  can hold several clusters.

The artist name is drawn from the first non-blank candidate.artist
across the group's members; a cluster whose members carry no artist name
renders no credit block at all (no empty applet, no link to a blank
page).

Wording deliberately diverges from the two existing callers'
"Art by <Name> - support them" copy: this credit describes a candidate
printing's artist, sourced from canonical Scryfall reference data,
before the voter has picked anything - not a claim about the voter's own
scanned card. "Illustration by <Name>" reads as a fact about the
artwork shown, not an assertion about the user's card.

The per-tile candidate.artist caption becomes redundant once its
cluster has its own credit, so it's dropped inside grouped clusters via a
new optional third parameter on the shared renderCandidateTile helper
(showArtistCaption, additive, defaults to true). Ungrouped candidates
are unaffected - they keep the existing plain-text caption unchanged and
do not get the applet treatment, since a flat grid can hold many
candidates by many different artists and a per-tile applet there would be
disproportionately heavy.

No wire-contract change: PrintingCandidate.artist already existed.
useGetArtistExternalLinksQuery is RTK Query and dedupes by its string
argument automatically, so multiple clusters sharing one artist collapse
to a single request with no code change needed.

Refs #385 for context (the MTGAC data-endpoint integration this reuses);
does not close it.

Test plan

• npx tsc --noEmit clean
• npx eslint on QuestionFeed.tsx (0 errors, 5 pre-existing
  no-img-element warnings on untouched lines, unrelated to this
  change)
• Jest: QuestionFeed.test.tsx (18 passed) and
  ArtistSupportLink.test.tsx (13 passed), run individually and
  combined (31 passed) - no order-dependence found (issue Test suite is order-dependent: leaked fetch-failure window trips the envelope across files (8 failures on master) #679)
• Playwright: QuestionFeed.spec.ts + QuestionFeedResponsive.spec.ts
  run against a live dev server with mocked network (42 passed),
  including the 44px tap-target guards (WCAG 2.5.5) and the
  illustration-grouping suite
• pre-commit run --files <touched files> (prettier + eslint) clean
• lsp_diagnostics - not available in this worktree
  (typescript-language-server not installed); tsc --noEmit +
  eslint + the test runs substitute, per this task's own guidance
• Full-suite context: a previous session's note that npm test
  carries 22 pre-existing failures in
  src/features/searchSettings/comparison.test.ts
  (Set.prototype.symmetricDifference unavailable on Node v20.20.2)
  was not re-investigated - confirmed unrelated to this change, not
  touched by it

Task-end checks (CLAUDE.md)

• Docs convention: docs/features/artist-support-links.md's
  "Surfaces" section updated in place (three → four), no dated section
  appended.
• Policy text dates: N/A, no on-site policy text touched.
• Wiki maintenance: this changes what a user sees (question feed now
  credits each illustration cluster's artist). Wiki: the "What's That
  Card?" user-guide page could note that illustration clusters now show
  an MTGAC artist credit - flagging for the merge-time checklist rather
  than editing directly from this worktree.
• Extractable-primitives ledger: no extraction here - this modifies
  QuestionFeed.tsx directly and reuses ArtistSupportLink unmodified,
  no new independent primitive produced.
• Constant renames: none.