v1.0.26: fix multi-selection reactivity, redesign icon, security review

Fix multi-selection: selectedKeys exposed as StateFlow, collected in
FilesScreen so checkboxes and highlights update correctly on every tap.
fileKey() made public so UI can check membership without ViewModel calls.

Icon: white cloud body with two cyan/teal circular sync arcs (AutoSync
style), deep blue-to-teal gradient background.

Security review clean: no hardcoded credentials, cleartext blocked by
network_security_config, allowBackup=false, path traversal guards in
place on both server responses and local resolution.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/src/main/kotlin/com/syncflow/ui/files/FilesScreen.kt

@@ -39,8 +39,9 @@ fun FilesScreen(
     val selectedPair by vm.selectedPair.collectAsState()
     val files by vm.files.collectAsState()
     val isDownloading by vm.isDownloading.collectAsState()
-    val isSelectionMode by vm.isSelectionMode.collectAsState()
-    val selectedCount by vm.selectedCount.collectAsState()
+    val selectedKeys by vm.selectedKeys.collectAsState()
+    val isSelectionMode = selectedKeys.isNotEmpty()
+    val selectedCount = selectedKeys.size
     val context = LocalContext.current
     val snackbarHostState = remember { SnackbarHostState() }
     val scope = rememberCoroutineScope()
@@ -252,12 +253,11 @@ fun FilesScreen(
                                 }
                             }
                             items(dirFiles, key = { "${it.syncPairId}_${it.relativePath}" }) { file ->
-                                val selected = vm.isSelected(file)
                                 FileRow(
                                     file = file,
                                     isInSubDir = dir.isNotEmpty() && !isSelectionMode,
                                     isSelectionMode = isSelectionMode,
-                                    isSelected = selected,
+                                    isSelected = vm.fileKey(file) in selectedKeys,
                                     vm = vm,
                                 )
                                 HorizontalDivider(