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/ROADMAP.md

@@ -28,6 +28,7 @@ Legend: ✅ done · 🚧 in progress · ⏳ not started · 💡 candidate (propo
 | Language / UI | Kotlin + Jetpack Compose | Mirrors the sibling app's proven, installed toolchain; no NDK |
 | Storage access | `MANAGE_EXTERNAL_STORAGE` first, SAF fallback | Google Play explicitly permits "All files access" for file managers; SAF keeps the app usable if the user declines |
 | Connectivity | Local-first (no `INTERNET` in v1) | Preserves the "nothing leaves the device" claim and keeps the Data Safety form trivial |
+| File sharing | Receives `SEND`/`SEND_MULTIPLE`, serves `GET_CONTENT`/`OPEN_DOCUMENT`, and deliberately does **not** register `ACTION_VIEW` | A file manager should be able to receive a file and to supply one, and neither costs a permission. Registering as a viewer as well would put Plain Files in the "open with" chooser the app itself uses, offering it back to itself |
 | Monetisation | One-time paid (€4.49; €2.29 launch price), set in the Console | No Billing client, no `BILLING` permission, no purchase code — which is what keeps "nothing leaves the device" true in the shipped APK. Free → paid is impossible after the first upload, so the price must be set beforehand. See [MONETISATION](MONETISATION.md) |
 
 ## Deliberately deferred (post-1.0)