Transition to the Meson build system and codebase refactoring

[vikdevelop]

This PR introduces a major refactor of Save Desktop:

🔑 Main changes

• Build system migration

  • Replaced the old custom installation logic with Meson.
  • Simplified project structure and aligned with GNOME/Flatpak standards.

• Project structure reorganization

  • The src folder is divided into gui and core subdirectories.
  • Centralized icons, .desktop file, schemas, and appdata under data/.
  • Replaced custom JSON translations with gettext .po files under po/.
  • Added Meson-based build definitions throughout.

• Codebase cleanup

  • Split large modules into smaller, maintainable components.
  • Improved readability and maintainability.
  • Minor increase in total LOC, but with clearer separation of concerns.

• UI/UX improvements

  • Minor polishing of the GTK4/libadwaita interface.
  • Improved consistency across dialogs and actions.

⚠️ Notes

• Flatpak and native environments: fully functional.
• Snap package: currently limited – GUI works, but saving/import is disabled due to Python subprocess restrictions. Legacy build will remain available until Snap support is resolved.

✅ Why this matters

This merge is the foundation for:

• Easier future development and contributions.
• Cleaner architecture aligned with GNOME project standards.