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.

README.md

@@ -37,8 +37,8 @@ Android 16 (API 36) from 31 August 2026.
 app/src/main/java/de/softwareapp_hb/filemanager/
 ├── core/      Pure-Kotlin engine (no Android imports!) — paths, sorting, formatting,
 │              search matching, copy/move planning, conflict policy
-├── data/      Storage backends (all-files access + SAF), volumes, bookmarks,
-│              file operations, archives, deletion
+├── data/      Storage backends (all-files access + SAF), volumes, shares coming in
+│              from other apps, file operations, archives, deletion
 ├── domain/    Browser + operation controllers, models, permission state
 ├── service/   Foreground service for long-running file operations
 ├── ui/        Compose theme, browser screen, components, ViewModels
@@ -139,8 +139,11 @@ UI.** Translated UI text has to be supplied in `values-<locale>/`.
 
 ## Known limitations
 
-- **"Open with" and Share only work in the all-files mode.** A SAF path is relative to the
+- **Sharing a file *out* only works in the all-files mode.** A SAF path is relative to the
   granted tree rather than a filesystem path, so it cannot be handed to `FileProvider`.
+  Receiving a share and being picked from another app's chooser work either way: the first
+  needs no filesystem path, and the second falls back to `RESULT_CANCELED` rather than
+  returning a URI the caller could not read.
 - **RAR, CAB, ISO and WIM are refused**, not supported — they need native code, which this
   project deliberately does not ship. (The Commons Compress jar contains no RAR classes at
   all.) **7z is different**: Commons Compress reads it in pure Java, so it *is* supported —