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

playwright.config.ts

@@ -0,0 +1,41 @@
+import { defineConfig, devices } from "@playwright/test";
+
+const baseURL = process.env.E2E_BASE_URL || "http://localhost:3000";
+const targetHost = process.env.E2E_TARGET_HOST || "192.168.50.61";
+const testUser = {
+  email: process.env.E2E_TEST_EMAIL || "admin@admin.com",
+  password: process.env.E2E_TEST_PASSWORD || "adminpass",
+};
+
+export default defineConfig({
+  testDir: "./e2e",
+  fullyParallel: false,
+  forbidOnly: !!process.env.CI,
+  retries: process.env.CI ? 2 : 0,
+  workers: 1,
+  reporter: [
+    ["list"],
+    ["html", { open: "never", outputFolder: "playwright-report" }],
+    ["json", { outputFile: "playwright-results.json" }],
+  ],
+  use: {
+    baseURL,
+    trace: "on-first-retry",
+    screenshot: "only-on-failure",
+    video: "retain-on-failure",
+  },
+  globalSetup: "./e2e/global-setup.ts",
+  globalTeardown: "./e2e/global-teardown.ts",
+  projects: [
+    {
+      name: "chromium",
+      use: { ...devices["Desktop Chrome"] },
+    },
+  ],
+  timeout: 120_000,
+  expect: {
+    timeout: 10_000,
+  },
+});
+
+export { baseURL, targetHost, testUser };