ArtistSupportLink: drop redundant adjacent name labels, nest disclosure inside the button (#747)

[WilfordGrimley]

Summary

Two related complaints against ArtistSupportLink (issue #747, building on #733):

1. Three of its four call sites rendered a separate "Art by " / "Illustration by " label immediately next to the applet, duplicating the name the applet's own page-link button already shows as its text. Removed the redundant label at all three (QuestionFeed.tsx's illustration-group credit and confirmed-artist credit, ArtistSection.tsx's Art-by line). The fourth call site (CardDetailedViewBody.tsx's metadata table) already relied on its row label and needed no change.
2. The expandable commerce-links/credit disclosure panel rendered as a sibling <div> below the collapsed link+toggle row rather than as part of the same widget. It now nests inside that row's shell, so it reads as part of the button rather than a separate floating block, across all four call sites (including ArtistSection.tsx's defaultExpanded case).

The panel nests inside the row's flex shell rather than literally inside the <a> element: this project builds via next export, so a defaultExpanded panel is serialised to static HTML at build time, and real anchor tags nested inside another anchor get split apart by the browser's HTML parser on load - which would have broken call site 3's always-expanded case on every page load.

Test plan

• npm run build (frontend/) - compiles cleanly, no new TypeScript/ESLint errors
• npm test (frontend/) - 73 suites / 676 tests pass, including ArtistSupportLink.test.tsx
• npx playwright test tests/QuestionFeed.spec.ts (chromium, standalone) - 28/28 pass
• npx playwright test tests/QuestionFeedResponsive.spec.ts (chromium, standalone) - 26/26 pass
• npx playwright test tests/ArtistSupportLink.spec.ts (chromium, standalone) - 2/2 pass
• npx playwright test tests/DisplayPage.spec.ts (chromium, standalone) - 32/33 pass; snip the one failure (display-empty-state timeout in an unrelated phone-viewport scroll test) reproduces identically on a clean origin/master checkout, confirmed pre-existing