build: add build-assets make target

Runs npm install + npm run build inside node:20-alpine Docker container,
mounting user/themes/intotheeast as the working directory.

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

Makefile

@@ -24,6 +24,12 @@ test: test-config test-post test-ui
 build:
 	docker compose build
 
+build-assets:
+	docker run --rm \
+	  -v $(PWD)/user/themes/intotheeast:/app \
+	  -w /app node:20-alpine \
+	  sh -c "npm install && npm run build"
+
 start:
 	docker compose up -d