ci: add GitHub Actions workflow + dev docs + test:all script

- .github/workflows/ci.yml: runs shared + server tests on push/PR
- docs/DEVELOPMENT.md: setup, run, test, architecture, status
- package.json: test:all script (shared + server suites)

package.json

@@ -27,7 +27,8 @@
     "eject": "react-scripts eject",
     "server:dev": "npm run dev --workspace server",
     "server:test": "npm test --workspace server",
-    "shared:test": "npm test --workspace shared"
+    "shared:test": "npm test --workspace shared",
+    "test:all": "npm run shared:test && npm run server:test"
   },
   "eslintConfig": {
     "extends": [