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

themes/intotheeast/js/main.js

@@ -0,0 +1 @@
+(()=>{})();

themes/intotheeast/js/map.js

@@ -0,0 +1 @@
+(()=>{})();

themes/intotheeast/js/src/main.js

@@ -0,0 +1 @@
+// placeholder — replaced in Task 2

themes/intotheeast/js/src/map.js

@@ -0,0 +1 @@
+// placeholder — replaced in Task 4