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/launch.json

@@ -0,0 +1,30 @@
+{
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "name": "Flask API (:5001)",
+      "type": "debugpy",
+      "request": "launch",
+      "module": "flask",
+      "cwd": "${workspaceFolder}",
+      "env": {
+        "FLASK_APP": "shopdb",
+        "FLASK_ENV": "development",
+        "FLASK_DEBUG": "1"
+      },
+      "args": ["run", "--port", "5001", "--no-reload"],
+      "jinja": true,
+      "justMyCode": false,
+      "console": "integratedTerminal"
+    },
+    {
+      "name": "Pytest (current file)",
+      "type": "debugpy",
+      "request": "launch",
+      "module": "pytest",
+      "cwd": "${workspaceFolder}",
+      "args": ["${file}", "-v"],
+      "console": "integratedTerminal"
+    }
+  ]
+}