feat: add comprehensive Playwright UI test suite

25 tests across auth (A1-A5), posting (P1-P5), validation (V1-V4),
tracker (T1-T5), and nav (N1-N5). Uses storageState for single login
per run. Replaces post-with-photo.spec.js with post.spec.js.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/ui/auth.setup.js

@@ -0,0 +1,22 @@
+// @ts-check
+const { test: setup } = require('@playwright/test');
+const path = require('path');
+const fs = require('fs');
+
+const USER = process.env.GRAV_TEST_USER;
+const PASS = process.env.GRAV_TEST_PASS;
+const AUTH_FILE = path.join(__dirname, '../.auth/user.json');
+
+setup('authenticate', async ({ page }) => {
+    if (!USER || !PASS) throw new Error('GRAV_TEST_USER and GRAV_TEST_PASS must be set in .env');
+
+    fs.mkdirSync(path.dirname(AUTH_FILE), { recursive: true });
+
+    await page.goto('/login');
+    await page.fill('input[name="username"]', USER);
+    await page.fill('input[name="password"]', PASS);
+    await page.click('button[type="submit"]');
+    await page.waitForSelector('text=successfully logged in', { timeout: 10_000 });
+
+    await page.context().storageState({ path: AUTH_FILE });
+});