fix(desktop): resolve root dir traversal and backend readiness in Tauri desktop app

[BerryUIKI]

Description

Fixes an issue where launching the standalone Tauri desktop executable \Berry AI Studio.exe\ displayed \ERR_CONNECTION_REFUSED\ (refused to connect to 127.0.0.1:8000).

Key Changes

1. Ancestors-based root directory detection: Replaced fixed 3-level parent traversal in \detect_root_dir()\ with \�ncestors()\ traversal to find the repository root directory regardless of how deeply nested the binary is located.
2. Fast-fail TCP check: Added a preliminary \TcpStream::connect_timeout\ in \is_backend_healthy()\ before issuing HTTP requests.
3. Premature exit detection & logging: Enhanced \wait_for_backend_ready()\ to check if the spawned Python child exited prematurely, and added logging to \�erry_desktop.log.
4. Defensive In-App Diagnostics: Replaced default WebView connection error with an in-window diagnostics card guiding the user to logs and offering a retry button.