Add Playwright e2e coverage and Node 20 pin

frontend/e2e/README.md

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+# Playwright E2E
+
+ResolutionFlow's Playwright suite is still intentionally lean, but it now covers the main routes and workflows most likely to break demos, onboarding, or day-one product usage.
+
+## What it covers
+
+- Public landing page
+- Redirect behavior for protected routes
+- UI login flow
+- Authenticated dashboard shell
+- Session history page
+- Session history filtering
+- Feedback page
+- Account settings page
+- Profile save flow
+- Flow library search
+- Start session directly from library search results
+- Resume incomplete session
+- Start and complete a troubleshooting session
+- Export preview for a completed session
+- Personal analytics page
+- Team analytics page
+- Shared sessions exports page
+- Public shared session page
+
+## How auth works
+
+- `auth.setup.ts` logs in through the backend API using the seeded team admin user
+- it writes a Playwright storage state file to `e2e/.auth/team-admin.json`
+- most authenticated specs reuse that state
+- `auth.spec.ts`, `public.spec.ts`, and `shared-session.spec.ts` explicitly clear storage so those tests stay truly unauthenticated
+
+## Local prerequisites
+
+1. PostgreSQL must be running
+2. `backend/venv` should exist with backend dependencies installed
+3. frontend dependencies must be installed
+4. Playwright Chromium must be installed once
+
+## First-time setup
+
+```bash
+cd frontend
+npm install
+npx playwright install chromium
+```
+
+## Run the suite
+
+```bash
+cd frontend
+npm run test:e2e
+```
+
+Useful variants:
+
+```bash
+npm run test:e2e -- --list
+npm run test:e2e:headed
+npm run test:e2e:ui
+npm run test:e2e:debug
+```
+
+## Backend boot behavior
+
+The Playwright config starts the backend automatically. It will:
+
+1. run Alembic migrations
+2. seed test users
+3. launch Uvicorn on `127.0.0.1:8000`
+
+Then it builds and serves the frontend via `vite preview` on `127.0.0.1:4173`.
+
+## Default test credentials
+
+- Email: `teamadmin@resolutionflow.example.com`
+- Password: `TestPass123!`
+
+Override them with:
+
+- `PLAYWRIGHT_TEST_EMAIL`
+- `PLAYWRIGHT_TEST_PASSWORD`
+
+## Common local issue
+
+If the suite fails before tests start with a database connection error, PostgreSQL is not running or the Playwright DB env vars do not point at a reachable database.