Diving partner / Diver – Add to favourites

[alpheios-one]

Version 1.7.6.7077

This improvement relates to:
Add buddy: Sort by dives & favourites (Original: submersion#638)"
#1237

At present, favourites can only be set when creating a dive.

In future, it should be possible to assign the favourite star under ‘Dive Partners’.

If you click on the star at the top, the diver will be marked as a favourite and pinned to the top.

[alpheios-one]

Follow-up while implementing this: buddy_detail_page.dart was already over the project's 800-line-per-file guideline before this change (874 lines), and adding the favorite-star button pushed it further (899, then back down to 881 after extracting a shared BuddyFavoriteButton widget to remove some duplication with the list tiles and the "Add buddy" picker).

As a small cleanup alongside this PR, we're additionally splitting the two largest self-contained chunks out of that file:

• the shared-dives card (~176 lines) into its own widget
• the dive-export logic (~102 lines, no UI) into a small service

Pure refactor, no behavior change -- just bringing the file back under the size guideline while we're already in there.