Add Playwright E2E test suite (25 tests across 4 specs)

- playwright.config.ts: headless CI setup with JSON/HTML reporters
- e2e/global-setup.ts: app reachability check before tests
- e2e/auth.spec.ts: 6 auth tests (sign-in, session, protected routes)
- e2e/compare.spec.ts: 8 API tests (validation, SSE flow, DB persistence)
- e2e/comparisons.spec.ts: 6 CRUD tests (list, detail, 404, view counts)
- e2e/user.spec.ts: 5 user tests (stats, pagination, session binding)
- e2e/helpers.ts: shared SSE parsing, auth, URL resolution utilities
- e2e/README.md: setup and run instructions
- AGENTS.md: updated with LLM provider notes and testing docs
- .gitignore: added playwright report/result directories
- package.json: added @playwright/test and test:e2e scripts

e2e/README.md

@@ -0,0 +1,76 @@
+# E2E Tests
+
+Playwright-based end-to-end tests covering auth, the compare API, comparisons CRUD, and user stats.
+
+## Setup
+
+```bash
+# Install dependencies (includes Playwright)
+npm install
+
+# Install Chromium browser for Playwright
+npm run test:e2e:install
+
+# Copy and fill in test credentials
+cp .env.test.example .env.test.local
+# Edit .env.test.local with your test user credentials
+```
+
+## Running Tests
+
+```bash
+# Run all tests (headless)
+npm run test:e2e
+
+# Run with headed browser (visible)
+npm run test:e2e:headed
+
+# Run with Playwright UI (step-through debugger)
+npm run test:e2e:ui
+
+# View the HTML report from last run
+npm run test:e2e:report
+```
+
+## Prerequisites
+
+The Comparaison Docker container must be running on the target host:
+
+```bash
+ssh bear@192.168.50.61 'cd /srv/compose/comparaison && docker compose up -d'
+```
+
+## Environment Variables
+
+| Variable | Default | Description |
+|---|---|---|
+| `E2E_HOST` | `192.168.50.61` | Host running the Docker container |
+| `E2E_BASE_URL` | `http://localhost:3000` | App URL for browser navigation |
+| `E2E_TARGET_HOST` | `192.168.50.61` | Host for API calls via Traefik |
+| `E2E_TEST_EMAIL` | `admin@admin.com` | Test user email |
+| `E2E_TEST_PASSWORD` | `adminpass` | Test user password |
+
+## Test Structure
+
+```
+e2e/
+  global-setup.ts      # Verifies app is reachable before tests
+  global-teardown.ts   # Cleanup after tests
+  helpers.ts           # Shared utilities (auth, SSE parsing, URL resolution)
+  auth.spec.ts         # Authentication flows (6 tests)
+  compare.spec.ts      # Compare API including SSE and DB persistence (8 tests)
+  comparisons.spec.ts   # List and detail views (6 tests)
+  user.spec.ts         # User stats and account (5 tests)
+```
+
+## CI Integration
+
+In CI (GitHub Actions, Gitea Actions), set environment variables and run:
+
+```bash
+npm install
+npm run test:e2e:install
+npm run test:e2e
+```
+
+The JSON results file (`playwright-results.json`) can be uploaded as a CI artifact.