fix(update): read ahead_by from GitHub compare API when status is ahead/diverged

The compare API URL format compare/{local_hash}...main returns
status='ahead' when main is ahead of the local commit. The count is
in the ahead_by field, not behind_by. The old code only read behind_by
which is always 0 in this case, causing the UI to show 'Up to date'
when actually several commits behind.

Also handle status='diverged' (both sides have unique commits) by
reading ahead_by for the remote-ahead count.

Frontend adds a hash comparison fallback: if behind_by is 0 but local
and remote commit hashes differ, show 'Behind main' instead of the
incorrect 'Up to date'.

Tests: _AheadCompareDownloader and _DivergedCompareDownloader mocks
for the two status paths.

py/routes/update_routes.py

@@ -517,7 +517,10 @@ class UpdateRoutes:
                     custom_headers={'Accept': 'application/vnd.github+json'}
                 )
                 if c_ok:
-                    behind_by = c_data.get('behind_by', 0)
+                    if c_data.get('status') in ('ahead', 'diverged'):
+                        behind_by = c_data.get('ahead_by', 0)
+                    else:
+                        behind_by = c_data.get('behind_by', 0)
 
             return version, changelog, behind_by, commit_date