Add browser Python editor with Pyodide, user auth, and workspace API

- FastAPI serves static UI, file CRUD under code/ and read-only lib/
- Pyodide worker runs Python and Jedi completions in the browser
- SQLite accounts: login/register, session cookies, superuser user management
- Optional EDITOR_API_KEY, AUTH_* env vars, .env.example
- Pipenv, pytest, Selenium smoke test, README

Made-with: Cursor

.env.example

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+# Copy to `.env` in the repo root (same directory as Pipfile). `.env` is gitignored.
+
+# Workspace tree: code/ (writable), lib/ (read-only); defaults to ./workspace under the repo root
+# WORKSPACE_ROOT=/home/you/projects/python-editor/workspace
+
+# If set, `/api/*` accepts this Bearer token (and still accepts session cookies if AUTH_ENABLED=true)
+# EDITOR_API_KEY=
+
+# --- User accounts (SQLite) ---
+# AUTH_ENABLED=true          # require sign-in for /api/* (except /api/auth/*)
+# AUTH_REGISTER_OPEN=true    # allow POST /api/auth/register
+# AUTH_DATABASE_PATH=./data/editor.db
+# AUTH_SESSION_DAYS=14
+# BOOTSTRAP_ADMIN_USERNAME=admin   # first-run only: create superuser if DB has zero users
+# BOOTSTRAP_ADMIN_PASSWORD=change-me-in-production
+
+# Base URL for `pipenv run test-selenium` (app must be running separately)
+# SELENIUM_BASE_URL=http://127.0.0.1:8080