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/extensions.json

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