build: add esbuild scaffolding and Docker build-assets target

package.json with esbuild ^0.21 and npm dependencies (maplibre-gl,
photoswipe, scrollama, fontsource). Placeholder src files and
compiled outputs committed. node_modules gitignored.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vgmzx8VTTTmCskSpQtsLTr

.gitignore

@@ -7,3 +7,4 @@
 /pages/02.post/*ui-test*/
 /config/plugins/git-sync.yaml
 /config/security.yaml
+/themes/intotheeast/node_modules/