v1.0.24: harmonious icon, recursive file watching, download-then-open, security fixes

Icon: three identical parallel arcing arrows (same bezier curve, same blue-to-teal
gradient #64C8FF→#32EDBB, same arrowhead geometry) — visually cohesive and clearly
visible against the near-black background.

FileWatchService: FileObserver is now recursive — watchDirRecursive() creates an
observer for each subdirectory at startup, and adds new watchers when CREATE events
produce new directories. Fixes files added to subdirectories not being detected.

FilesViewModel: openFile/shareFile now fall back to download-then-open when the file
is absent locally. AccountRepository + ProviderFactory injected; downloads to
context.cacheDir/syncflow_open/ with isDownloading state. Path traversal guard added
(reject relativePath containing ".."). file_paths.xml gains cache-path entry.

WebDavProvider: path-traversal guard in parsePropfind — skip any server-returned
filename containing "..", "/" or "\". Replace android.util.Log with Timber so debug
logs are stripped from release builds.

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

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

@@ -33,6 +33,7 @@ fun FilesScreen(
     val pairs by vm.pairs.collectAsState()
     val selectedPair by vm.selectedPair.collectAsState()
     val files by vm.files.collectAsState()
+    val isDownloading by vm.isDownloading.collectAsState()
     val context = LocalContext.current
     val snackbarHostState = remember { SnackbarHostState() }
     val scope = rememberCoroutineScope()
@@ -182,6 +183,31 @@ fun FilesScreen(
             }
         }
 
+        if (isDownloading) {
+            Box(
+                modifier = Modifier
+                    .fillMaxWidth()
+                    .align(Alignment.BottomCenter),
+            ) {
+                Surface(
+                    color = MaterialTheme.colorScheme.surfaceVariant,
+                    tonalElevation = 4.dp,
+                    modifier = Modifier.fillMaxWidth(),
+                ) {
+                    Row(
+                        modifier = Modifier
+                            .fillMaxWidth()
+                            .padding(horizontal = 16.dp, vertical = 10.dp),
+                        verticalAlignment = Alignment.CenterVertically,
+                        horizontalArrangement = Arrangement.spacedBy(12.dp),
+                    ) {
+                        CircularProgressIndicator(modifier = Modifier.size(18.dp), strokeWidth = 2.dp)
+                        Text("Downloading for preview…", style = MaterialTheme.typography.bodySmall)
+                    }
+                }
+            }
+        }
+
         SnackbarHost(
             hostState = snackbarHostState,
             modifier = Modifier.align(Alignment.BottomCenter),