Security: guard against path traversal from hostile remotes

WebDAV already sanitizes server-supplied names, but SFTP passed entry.name
through unfiltered, and the engine had no central guard — a malicious or
compromised remote could return '../../x' and (on the JavaFile backend) write
outside the sync root.

- SyncEngine: isUnsafeSyncPath() rejects empty, absolute, and any '..'-segment
  path; every file is checked before any read/write/delete (covers all providers).
- SftpProvider.listFiles: drop '.'/'..' and names containing path separators.
- PathSafetyTest covers traversal, backslash, absolute, and empty cases.

app/src/main/kotlin/com/syncflow/domain/sync/SyncEngine.kt

@@ -111,6 +111,14 @@ class SyncEngine @Inject constructor(
             allPaths.map { rel ->
                 async {
                     semaphore.withPermit {
+                        // Defense-in-depth against a malicious/compromised remote returning a
+                        // path that escapes the sync root (e.g. "../../evil"). Skip rather than
+                        // write outside pair.localPath / pair.remotePath.
+                        if (isUnsafeSyncPath(rel)) {
+                            Timber.w("SyncEngine: skipping unsafe path for pair ${pair.id}: $rel")
+                            logEvent(pair.id, SyncEventType.FILE_SKIPPED, rel, "unsafe path", 0)
+                            return@withPermit FileOutcome(skipped = 1)
+                        }
                         val local = localFiles[rel]
                         val remote = remoteFiles[rel]
                         val known = knownStates[rel]
@@ -363,6 +371,19 @@ internal fun syncDecide(
 
 enum class SyncDecision { UPLOAD, DOWNLOAD, DELETE_LOCAL, DELETE_REMOTE, CONFLICT, SKIP }
 
+/**
+ * True if a relative sync path is unsafe to act on — empty, absolute, or containing a ".."
+ * segment that would let a hostile remote escape the sync root via path traversal. Applied to
+ * every path before any file operation as defense-in-depth (WebDAV already filters names at the
+ * parser; SFTP and any future provider are covered here).
+ */
+internal fun isUnsafeSyncPath(rel: String): Boolean {
+    if (rel.isBlank()) return true
+    val normalized = rel.replace('\\', '/')
+    if (normalized.startsWith("/")) return true
+    return normalized.split('/').any { it == ".." }
+}
+
 data class SyncResult(
     val uploaded: Int = 0,
     val downloaded: Int = 0,