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>

playwright.config.js

@@ -1,11 +1,22 @@
 // @ts-check
-const { defineConfig } = require('@playwright/test');
+const { defineConfig, devices } = require('@playwright/test');
 
 module.exports = defineConfig({
     testDir: './tests/ui',
     globalSetup: './tests/global-setup.js',
     timeout: 30_000,
     retries: 0,
+    projects: [
+        { name: 'setup', testMatch: /auth\.setup\.js/ },
+        {
+            name: 'chromium',
+            use: {
+                ...devices['Desktop Chrome'],
+                storageState: 'tests/.auth/user.json',
+            },
+            dependencies: ['setup'],
+        },
+    ],
     use: {
         baseURL: process.env.GRAV_BASE_URL || 'http://localhost:8081',
         headless: true,