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/global-setup.ts

@@ -0,0 +1,32 @@
+import type { FullConfig } from "@playwright/test";
+import { baseURL } from "./playwright.config";
+
+/**
+ * Global setup — runs once before all tests.
+ * Verifies the app is reachable before running E2E suite.
+ */
+export default async function globalSetup(_config: FullConfig) {
+  const url = baseURL.replace("localhost", process.env.E2E_HOST || "192.168.50.61");
+  console.log(`[E2E Setup] Checking app at: ${url}`);
+
+  let attempts = 0;
+  while (attempts < 10) {
+    try {
+      const res = await fetch(url, { signal: AbortSignal.timeout(5000) });
+      if (res.ok || res.status === 401) {
+        // 401 means the server is up (auth is working), 200 means homepage
+        console.log(`[E2E Setup] App reachable (HTTP ${res.status})`);
+        return;
+      }
+    } catch {
+      // ignore and retry
+    }
+    attempts++;
+    await new Promise((r) => setTimeout(r, 2000));
+  }
+
+  throw new Error(
+    `[E2E Setup] App not reachable at ${url} after ${attempts} attempts. ` +
+    "Ensure the Docker container is running: `docker compose up -d`"
+  );
+}