site: list mosaic tiles for /finds/ + home grid

Adds a build-time mosaic generator (scripts/build-list-mosaics.mjs, sharp)
that composites a single tile image per superpost from its referenced
finds. Wired via predev/prestart/prebuild hooks; output lives in
src/generated/ (gitignored). The /finds/ index and home grid both render
the mosaic when present and fall back to text-only tiles otherwise.

package.json

@@ -4,6 +4,10 @@
   "version": "0.1.0",
   "private": true,
   "scripts": {
+    "build:mosaics": "node scripts/build-list-mosaics.mjs",
+    "predev": "npm run build:mosaics",
+    "prestart": "npm run build:mosaics",
+    "prebuild": "npm run build:mosaics",
     "dev": "astro dev",
     "start": "astro dev",
     "build": "astro check && astro build",
@@ -16,6 +20,7 @@
     "@astrojs/rss": "^4.0.12",
     "@astrojs/sitemap": "^3.6.0",
     "astro": "^5.1.1",
+    "sharp": "^0.34.0",
     "typescript": "^5.7.2"
   }
 }