Receive shares and serve file requests, so other apps can reach this one

The manifest had nothing but a LAUNCHER filter, so no other app could hand Plain Files anything. It now registers for ACTION_SEND and SEND_MULTIPLE (a bar over the browser: navigate to the folder, then Save here) and for GET_CONTENT and OPEN_DOCUMENT, where tapping a file hands it back through the existing FileProvider.

Both reuse the browser instead of a second screen, because both are navigate-to-a-folder with a different verb at the end. data/ gains one port, IncomingSource, to keep a ContentResolver out of the ViewModel the way DeleteRequestFactory keeps one out of the delete path. Shared files are named from the sender OpenableColumns, allocated by the same NameAllocator a paste uses, and deleted again if they cannot be read rather than left as zero-byte stubs. While providing, a tap on a file chooses it but a tap on a folder still navigates, or the user could never reach the file.

ACTION_VIEW is deliberately not registered: the app already hands files out through the system chooser, and registering as a viewer would offer Plain Files back to itself. No new permissions - the merged release manifest still declares no INTERNET.

docs/STORE_LISTING.md

@@ -97,6 +97,12 @@ ANY SIZE OF SCREEN
 • Pictures preview at full size beside the list, with their details
 • The same layout on a phone, a 7-inch tablet and a 10-inch tablet
 
+WORKS WITH YOUR OTHER APPS
+• Share a file to Plain Files from anywhere, then pick the folder to drop it in
+• Choose Plain Files when another app asks you for a file
+• Search a whole folder tree, not just the folder you are looking at
+• Delete, rename or move by mistake? Undo it
+
 MANAGE YOUR FILES
 • Copy, move, rename and delete, one item or many at once
 • Create folders and edit text files in the built-in editor