Fix list thumbnail icon backgrounds

[ewlarson]

Summary

• restore list-view resource-class placeholders to the icon-only gradient required by issue List view: left hand inset should be thumbnail or default icon #125
• preserve current gallery and map thumbnail behavior
• add regression coverage across URL selection, rendered markup, SSR proxying, and backend SVG output

Tests

• frontend focused Vitest suite: 53 passed
• backend resource-thumbnail suite: 19 passed
• frontend production build
• targeted ESLint, Prettier, and Ruff checks

Repository-wide frontend lint and format checks still report pre-existing failures in unrelated files; all changed files pass targeted checks.