fix(ui): restore Install and Exit buttons screen on startup

[MedCy1]

Fix: Prevent auto-installation and restore start screen buttons

What does this PR do?
This PR fixes a bug where the installation process started automatically immediately after the application launch animation. It restores the intended behavior where the user is presented with a "Ready to install" screen and must manually click "Install" to proceed.

Key Changes:

• Prevent Auto-start: Removed the automatic call to _start_installation() inside on_fade_in_complete.
• UI Layout: Grouped "Install" and "Exit" buttons into a dedicated buttons_frame for better alignment (side-by-side).
• State Management:
  • The progress section is now hidden by default (pack_forget).
  • Clicking "Install" now correctly hides the buttons and reveals the progress section.
• UX Improvement: Updated the initial status label from "Preparing..." to "Ready to install".

How to test:

1. Launch the installer (python app.py).
2. Verify that the installation does not start automatically.
3. Verify that "Install" and "Exit" buttons are visible and correctly aligned.
4. Click "Install" and verify the transition to the progress bar / installation process.