Add development setup guide and shared VS Code config

Clone-to-first-change onboarding, plus .vscode launch/tasks/extensions
so F5 debugs the backend and a task runs both dev servers. Fix the
README manual path to run the backend on 5001 (the frontend proxies
there; the default 5000 loaded nothing).

.vscode/extensions.json

@@ -0,0 +1,9 @@
+{
+  "recommendations": [
+    "ms-python.python",
+    "ms-python.vscode-pylance",
+    "Vue.volar",
+    "dbaeumer.vscode-eslint",
+    "ms-azuretools.vscode-docker"
+  ]
+}