Port Calculate activity to GTK4

This commit fully ports the Calculate activity to GTK4 and Sugar4.

- Port `shareable_activity.py` with updated DBus and Sugar4 base classes.
- Rework `layout.py` to use GTK4 box packing (`append`, `set_child`) instead of deprecated containers like `Gtk.Table`.
- Replace legacy styling (`modify_bg`, `modify_font`) and `Gtk.EventBox` usage with a modern `Gtk.CssProvider` to handle backgrounds, colors, and font rendering.
- Ensure proper CSS specificity (`background-image: none;`) to override GTK4's native `.toggle` styles for the toolbar interface.
- Refactor `toolbars.py` to gracefully render GTK4 toolbar items and correctly update text fallbacks when SVG icons are invalid.
- Port `calculate.py` to use `Gtk.EventControllerKey` for keystrokes and update clipboard operations to use GTK4 async mechanisms.
- Include `main.py` standalone bootstrapper for isolated GTK4 testing.

Author: Inuth0603