Fix release signing and unify "+" upload with the share-intent path

- Release builds now sign with a dedicated keystore (local
  android/key.properties or CI secrets, both gitignored) instead of each
  machine's own debug key, so a downloaded release APK can actually
  update a previous install instead of failing with "App not installed"
  (mismatched signature).
- The Files tab's "+" -> "Upload File" now goes through the same
  ShareUploadView destination picker and background foreground-service
  upload as receiving a file via Android's "Share to..." sheet, instead
  of a separate blocking in-app-only upload path. Removed the
  now-unused ServerProvider/NextcloudService.uploadFileFromPath.

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

android/.gitignore

@@ -13,3 +13,4 @@ GeneratedPluginRegistrant.java
 key.properties
 **/*.keystore
 **/*.jks
+RELEASE_SIGNING_SECRETS.txt