update landing page

This MR includes design and usability adjustments for the landing page.

There is a widget for recent activity only visible to logged in users. It should show recent modification or creation activity by the user's colleagues (members of the same groups)

The recent activity widget has been temporarily removed and will be handled in a separate branch.

I also changed the page background from white to light gray and the primary text color from black to dark grey. This ensures better contrast between cards and background and makes the text more pleasant to read. These changes still need to be implemented for the whole application.

Closes #104 (closed)