What's That: replace the MTGAC applet with a compact link to the artist's MTGAC page

[WilfordGrimley]

Summary

• ArtistSupportLink used to always stack a page-link button, up to five commerce buttons, a signature badge, and a credit line - never fewer than 2 rows, up to ~8. It's a same-origin link-out to this project's own cache-only backend endpoint, not a third-party embed (issue MTG Artist Connection data-endpoint integration #385's declined-embed posture is unaffected).
• Default render is now one line: the artist page link plus a disclosure toggle (chevron-down/chevron-up, aria-expanded). Commerce links, the signature badge, and the "Source: MTG Artist Connection" credit move into a panel that only mounts once the user expands it - every link that existed before is still reachable, just not paid for in vertical space by default.
• Confirmed call sites (still four, none deleted, none added): QuestionFeed.tsx (illustration-group credit and the confirmed-artist-vote credit), ArtistSection.tsx (/editor rail), CardDetailedViewBody.tsx (card-detail modal's metadata table). None of these files needed a code change except ArtistSection.tsx.
• ArtistSection.tsx (the /editor rail's Artist accordion pane) opts into a new optional defaultExpanded prop so it starts open - that surface is a dedicated pane with room for the full applet, and its own Playwright coverage already asserted the credit line renders without interaction. Every other caller is space-constrained next to whatever the surface is actually about, so the default stays collapsed. This is a density prop on an existing component, not a new mechanism, so no separate doc ships with it.
• QuestionFeed.tsx itself is untouched - both its call sites already pass only artistName, which still composes correctly against the new default.

Test plan

• npx jest - 73 suites / 674 tests passing
• npx playwright test - full suite (345 tests) run; only pre-existing, non-deterministic failures unrelated to this change (icon-font aria-snapshot rendering, phone-viewport scroll timing) - different failures on different runs, none touching ArtistSupportLink/question-feed/editor-artist-section coverage
• ArtistSupportLink.spec.ts, DisplayPage.spec.ts:931, and the full QuestionFeed.spec.ts (including a new test asserting the applet's expansion never overlaps the pinned reference image, per SPEC-wtc-rebuild.md's container-pinning amendment) all pass in isolation, repeatably
• Manually verified in a real browser at 1280px and 390px container widths: collapsed default renders one line, the toggle expands to reveal the credit (and, per unit tests, commerce links/badge when present), and the expansion never covers or displaces the reference card image
• Manually verified the /editor rail's Artist section starts expanded with the credit visible, matching prior behavior