diag: add /test-structured-output endpoint

diag: detect browser_use version and test explicit Browser API

diag: add /test-browser raw Playwright endpoint

diag: add /test-agent endpoint with full traceback

fix: switch to gemini-2.5-flash-lite, expand retry pattern (app.py)

diag: add /test-llm endpoint to isolate LLM connection issue

fix: update model IDs to current OpenRouter free models (app.py)

fix: add GET / root route so HF Space iframe renders

feat: initial KAZE browser-use HF Space