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>

lib/providers/server_provider.dart

@@ -1574,24 +1574,6 @@ class ServerProvider extends ChangeNotifier with WidgetsBindingObserver {
     _startCacheRefreshTimerIfNeeded();
   }
 
-  Future<bool> uploadFileFromPath(
-    String name,
-    String localFilePath, {
-    void Function(int sent, int total)? onProgress,
-  }) async {
-    if (_service == null) return false;
-    final success = await _service!.uploadFileFromPath(
-      _currentFolderPath,
-      name,
-      localFilePath,
-      onProgress: onProgress,
-    );
-    if (success) {
-      await refreshData();
-    }
-    return success;
-  }
-
   Future<bool> deleteItem(String itemPath) async {
     if (_service == null) return false;
     final success = await _service!.deleteItem(itemPath);