fix: generate the missing seed art images (broken thumbnails)

The art manifest referenced /art/*.png files that never existed; the SPA
fallback returned index.html for them, so the library thumbnails rendered
as broken images. Generate 5 atmospheric B&W placeholder scenes
(tools/gen-art-placeholders.mjs, npm run gen:art) into public/art/.
tests/tools/art-assets.test.js ties every manifest entry to a real PNG.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -11,6 +11,7 @@
     "test:watch": "vitest",
     "gen:scenario": "node tools/md-to-scenario.mjs",
     "gen:icons": "node tools/gen-icons.mjs",
+    "gen:art": "node tools/gen-art-placeholders.mjs",
     "ios:add": "npm run build && cap add ios",
     "ios:sync": "npm run build && cap sync ios",
     "ios:open": "cap open ios",