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/res/xml/file_paths.xml

@@ -3,4 +3,5 @@
     <external-path name="external_storage" path="." />
     <external-files-path name="external_files" path="." />
     <files-path name="internal_files" path="." />
+    <cache-path name="syncflow_cache" path="syncflow_open/" />
 </paths>