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.

.gitignore

@@ -2,6 +2,9 @@
 dist/
 .output/
 
+# generated assets
+src/generated/
+
 # generated types
 .astro/