Make delete, rename and move undoable

A delete is no longer a removal: it is a move into a hidden .plainfiles-trash folder on the same volume, which costs a rename and keeps the item recoverable. The snackbar offers Undo for delete, rename and a pasted cut; the handle is cleared as soon as it is used.

domain/Trash carries four deliberate limits: the trash lives on the users own volume, so it counts against their free space until it expires (24 hours, purged opportunistically, no startup hook and no empty-trash chore); a name taken again in the meantime is never overwritten, so an item returns beside it; a delete through Androids own dialog is not staged because the system owns it; and each item is staged in its own slot so two files sharing a name cannot overwrite each other on the way in. When a pasted move landing names cannot be established with confidence, no undo is offered rather than risking the wrong file moving back.

CHANGELOG.md

@@ -38,6 +38,24 @@ upload needs a **higher `versionCode`** than every previous upload.
 
 ### Added
 
+- **Undo for delete, rename and move.** A delete is now a *move* into a hidden
+  `.plainfiles-trash` folder on the same volume rather than a removal, so the snackbar can
+  offer to put it back; a rename and a pasted cut record how to reverse themselves. Four
+  deliberate limits, all carried in `domain/Trash`: the trash sits on the user's own volume,
+  so it counts against their free space until it expires (24 hours, purged opportunistically
+  so there is no startup hook to forget and no "empty trash" chore); a name taken again in
+  the meantime is never overwritten, so an item returns *beside* it under a free name; a
+  delete that went through Android's own dialog is not staged, because the system owns that
+  one and its trash with it; and when a pasted move's landing names cannot be established
+  with confidence, no undo is offered at all rather than risking putting the wrong file
+  back. The undo handle is cleared the moment it is used, so a second tap cannot replay it.
+
+- **Keep the browsed folder and view settings across process death.** `SavedStateHandle`
+  carries the folder plus the sort key, direction, view mode and hidden-files setting. The
+  search query, the selection and the clipboard are deliberately *not* restored — a
+  restored query would hide the folder's contents on launch with no visible cause, and a
+  paste that survived a reboot would be a nasty surprise.
+
 - **7z extraction.** Commons Compress already contained a pure-Java 7z reader
   (`SevenZFile`), so this needed no new native code and no library beyond the XZ one the
   `.xz` fix above required — the docs had simply assumed the format was off-limits and said