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/tasks.json

@@ -0,0 +1,43 @@
+{
+  "version": "2.0.0",
+  "tasks": [
+    {
+      "label": "Backend: flask run (:5001)",
+      "type": "shell",
+      "command": "${workspaceFolder}/venv/bin/flask run --port 5001",
+      "options": {
+        "cwd": "${workspaceFolder}",
+        "env": { "FLASK_APP": "shopdb" }
+      },
+      "windows": {
+        "command": "${workspaceFolder}\\venv\\Scripts\\flask.exe run --port 5001"
+      },
+      "isBackground": true,
+      "problemMatcher": []
+    },
+    {
+      "label": "Frontend: npm run dev (:5173)",
+      "type": "shell",
+      "command": "npm run dev",
+      "options": { "cwd": "${workspaceFolder}/frontend" },
+      "isBackground": true,
+      "problemMatcher": []
+    },
+    {
+      "label": "Dev site (backend + frontend)",
+      "dependsOn": [
+        "Backend: flask run (:5001)",
+        "Frontend: npm run dev (:5173)"
+      ],
+      "dependsOrder": "parallel",
+      "problemMatcher": []
+    },
+    {
+      "label": "Check: naming + tests + build",
+      "type": "shell",
+      "command": "bash scripts/check-naming-and-style.sh && venv/bin/python -m pytest tests/ -q && cd frontend && npx vitest run && npm run build",
+      "options": { "cwd": "${workspaceFolder}" },
+      "problemMatcher": []
+    }
+  ]
+}