Add fullscreen in-app image view, no stretch, sharing the editor's in-app/external toggle

Core.py

@@ -205,6 +205,11 @@ class AppConfig:
     ListIconSize: int = 32
     FontSize: int = 14  # entry name text, list and grid alike; metadata/header text scales with it
     ToolbarHamburger: bool = False  # collapse the toolbar's buttons into one overflow menu
+    # Double-click on a text/config/markup file: True opens it in SaraFem's own
+    # viewer/editor (Editor.py); False (matches pre-existing behaviour) hands it
+    # to xdg-open instead. Either way, the per-entry "Open" menu item does the
+    # opposite of this default, so both are always one click away.
+    OpenFilesInApp: bool = False
 
     # Batch-rename pattern for multi-select rename, syntax modeled on
     # SaraSortd's own placeholder characters (see ~/Applications/SaraSortd's