Remove the drag-right-to-go-back gesture

The screen no longer follows a horizontal drag. Back is the arrow at the
top left and the platform's own edge gesture, both unchanged.

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

EXPLORER.md

@@ -214,7 +214,7 @@ which highlighting is on -- see "Numbers to measure".
 `FilesScreen` is composed **on top of** the session in the same `Box`, and
 the session stays composed under it: its event stream keeps flowing, its
 scroll position and draft stay where they were, and returning from a file
-costs nothing. Back -- the button, the platform gesture and `swipeBack` --
+costs nothing. Back -- the button and the platform gesture --
 clears `files` when it is set and goes to the list otherwise. Inside the
 explorer the same back steps one level: editor → viewer (with the unsaved
 question), viewer → listing, listing → parent directory it came from, and