Improve UI layout dumping, status bar, and mouse mode

Add mouse_mode state and an optional mouse mode selector in preferences/status bar. Implement synthetic-item registration for ImGui test-engine layout dumps (controlled by env flags), make window collection more robust, increase item gather reserve and include has_id in item output. Refactor main viewer UI: setup main window geometry, embed a compact status bar with image/view info and mouse-mode combo, improve fit-to-window behavior, and add more layout-dump synthetic items for placeholders. Enable ImGui viewports support and tweak style for platform windows; log startup load failures and print image load/upload errors to stderr. Adjust testing/tools: bump default layout-depth to 8 in the test runner, optionally render item debug labels into generated SVGs, and make the upload smoke-test detect missing image loads (fail with reason image_not_loaded).

Author: ssh4net