Clean up build tooling and Dockerfile

- Dockerfile cross-compiles Go for the target platform (amd64/arm64
  without emulation), uses templ pinned in go.mod, runs as uid 1000 on
  alpine and lets FRONTEND_IMAGE pin the frontend version
- justfile: tools via go tool, `test` (unit) and `test-integration`
  (uses the active docker context), no hardcoded paths, no tailwind or
  frontend recipes
- Removed app.yaml, init.sh, tailwind.config.js and the tracked .idea
  folder

.dockerignore

@@ -14,18 +14,12 @@ compose.yaml
 *.pprof
 main
 conf.yaml
-output.css
-tailwindcss
 .env
 
-# Node.js (frontend build artifacts)
-node_modules
-package.json
-package-lock.json
-
-# Test files
+# Test files and fixtures
 *_test.go
-integration_test.go
+internal/testutil
+*_dump.sql
 
 # OS
 .DS_Store