Fix WebDAV upload of non-ASCII/special filenames (URL + MOVE header encoding)

Volume test (100 files) surfaced it: files with non-ASCII names (e.g. 'naïve
café.txt') failed to upload — url() built a raw string, so the MOVE Destination
header carried non-ASCII chars that OkHttp rejects. Now url() percent-encodes
each path segment via HttpUrl.addPathSegments (also covers '&', spaces, CJK).
Regression test specialAndNonAsciiNames_upload added.

app/src/main/kotlin/com/syncflow/data/providers/webdav/WebDavProvider.kt

@@ -181,7 +181,13 @@ open class WebDavProvider(protected val account: CloudAccount) : CloudProvider {
         }
     }
 
-    protected fun url(path: String) = "$baseUrl/${path.trimStart('/')}"
+    // Build a properly percent-encoded URL. addPathSegments encodes each segment (spaces,
+    // ampersands, and — critically — non-ASCII like "café"), which keeps OkHttp from rejecting
+    // non-ASCII in the WebDAV MOVE "Destination" header and avoids malformed request URLs.
+    protected fun url(path: String): String {
+        val base = baseUrl.toHttpUrlOrNull() ?: return "$baseUrl/${path.trimStart('/')}"
+        return base.newBuilder().addPathSegments(path.trimStart('/')).build().toString()
+    }
 
     private fun parsePropfind(xml: String, parentPath: String, dropFirst: Boolean = true): List<RemoteFile> {
         val results = mutableListOf<RemoteFile>()