Add files/folders/both filter and folder-navigation slide animation

The Files controls bar now scrolls horizontally to fit a new segmented
toggle (files & folders / files only / folders only), persisted per
account like the other browsing prefs.

Navigating into or out of a folder now slides the listing in (from the
right when going deeper, from the left when going back), staggered
slightly per row. This is a per-item animation rather than a real page
transition, since folder navigation swaps ServerProvider's path stack in
place rather than pushing Navigator routes - true OS-level predictive
back would need that to change.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

lib/services/account_store.dart

@@ -35,6 +35,7 @@ class AccountStore {
     'ui_sort_field',
     'ui_sort_ascending',
     'ui_folder_sort',
+    'ui_files_type_filter',
     'ui_cache_policy',
     'ui_cache_interval_minutes',
   ];