feat: persistent DB host config, async DB ops, Exit button, test connection

- Save/load DB host from config.json (works both in dev and built .exe)
- All DB operations moved to background threads to prevent UI blocking
- Connection timeout set to 5s to avoid long hangs on unreachable host
- Added Test Connection button to Settings popup
- Added red Exit button fixed to bottom-right corner of main screen
- Updated DatabaseApp.spec with full Kivy deps (sdl2, glew, angle) and hidden imports
- PyInstaller-aware base path using sys.frozen for config.json persistence

.gitignore

@@ -8,7 +8,6 @@ venv/
 env/
 ENV/
 *.egg-info/
-dist/
 build/
 *.egg