Add development setup guide + shared VS Code config

New docs/DEVELOPMENT-SETUP.md: clone-to-first-change onboarding
(Docker fast path, manual venv+Node daily driver, VS Code, the dev
loop, first-change pointer at the plugin lab, troubleshooting). Ship
.vscode/ launch/tasks/extensions so F5 debugs the backend on 5001 and
a task runs both servers; personal settings.json stays ignored. Fix
the README manual path - it ran the backend on the default 5000, but
the frontend dev server proxies to 5001, so nothing loaded; also add
the plugin upgrade-all step and a VS Code pointer.

.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"
+    }
+  ]
+}