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

.gitignore

@@ -3,13 +3,8 @@ tmp
 .idea
 *templ.go
 conf.yaml
-output.css
 compose.yaml
-tailwindcss
 .env
-node_modules
-package.json
-package-lock.json
 cpu.pprof
 main