fix: side-by-side diff scroll desync on hidden-tab rebuild and tab switch

[JC-Chung]

Why

OnLoaded + FindDescendantOfType<ScrollViewer>() assumed the control
template was already applied, which broke in two cases:

1. Presenter rebuilt while its container is hidden (IsVisible=false),
   e.g. toggling the shared Preferences.UseSideBySideDiff from the
   other LOCAL/HISTORY tab — Loaded fires but template application
   (tied to layout) is skipped for hidden subtrees.
2. Commit-detail tabs (Info/Changes/Files) reuse the same presenter
   instance across tab switches — clearing _scrollViewer to null in
   OnUnloaded permanently lost the binding since OnApplyTemplate
   only runs once per instance.

Fix

Move ScrollViewer lookup + offset binding into OnApplyTemplate
(matching MergeConflictTextPresenter), force ApplyTemplate() in
OnLoaded, and stop resetting _scrollViewer in OnUnloaded.

Test

1. Toggle Side-by-Side in LOCAL/HISTORY tab, switch to the other — stays synced.
2. Commit detail: Changes → Info/Files → back to Changes — stays synced.

[love-linger]

It's not necessary add move code to OnApplyTemplate after manually calling ApplyTemplate() in OnLoaded.

All we need is adding a single line ApplyTemplate(); after calling base.OnLoaded(e)

[love-linger]

Tested it manually. Looks good to me.