Improve mobile UX

Several Mobile UX aspects lead to confusion:

• No page name when in fullscreen
• Filtering by scope isn't grouped with other filters
• Back button is confused with undo button

To improve all these aspects, a new UX for fullscreen mobile is submitted to the City of Lausanne: https://www.figma.com/design/pD8isgfNWmIvpWciYQlnD7/decidim-Geo-v3-(mobile)?node-id=1370-231&t=ff53TwhRFzms0TJ0-1

tasks

• Add the header with the page name
  • add pagename option in configuration store
  • fetch serverside current page name
  • trunk page name length if it is too big
  • handle click surface of exiting fullscreen button
• Filter by scope grouped with other filters (already done in #150 (closed))
• Back button is confused with undo button.