fix(css): Use flexbox to align editor container and search dialog

[mejo-]

This fixes stickyness of the search dialog to the bottom with long documents.

We have 2 or 3 elements in app-content-details: the page title, the editor, and optionally the search dialog. Page title and editor should align to the top, search dialog shall align to the bottom. Scroll container shall be in the editor.

First, the editor didn't take all space, so with a short document, the search dialog didn't align to the bottom. This was tried to fix by e918085, which had other unwanted side effects. So another attempt was bfb69b3. But the scroll container was wrong, so the search dialog moved out of the viewport with this approach. Instead of playing around more with position: sticky I decided to migrate it to flexbox instead.

I tested view and edit mode with sheet and full page width and with short documents and long documents. I also checked the loading skeletons for page header and page content.

🏁 Checklist

• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
• Sign-off message is added to all commits