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>

.gitignore

@@ -15,6 +15,7 @@ user/plugins/
 node_modules/
 test-results/
 playwright-report/
+tests/.auth/
 
 # OS
 .DS_Store