fix: resolve android read-only file system error on sync

Android 10+ (API 29+) enforces Scoped Storage via the Storage Access Framework (SAF),
meaning that standard `std::fs` operations cannot write to user-selected virtual URIs,
causing a `Read-only file system (os error 30)` panic when the backend tried to initialize
the hidden `.sync` directory inside the workspace.

- Relocated the Automerge `.am` metadata files into the Tauri application's native,
  read-write enabled config directory (`app_config_dir()`).
- Added MD5 hashing of the workspace path to ensure different workspace trees
  get their own isolated sub-folder of sync states to prevent cross-workspace collisions.

Co-authored-by: Keshav-writes-code <95571677+Keshav-writes-code@users.noreply.github.com>

Author: google-labs-jules[bot]

apps/app/src-tauri/Cargo.lock

@@ -47,6 +47,7 @@ rand = "0.9.2"
 dirs = "6.0.0"
 notify = "8.2.0"
 notify-debouncer-full = "0.6.0"
+md5 = "0.8.0"
 
 [profile.dev]
 incremental = true # Compile your binary in smaller steps.

apps/app/src-tauri/Cargo.toml

@@ -19,12 +19,18 @@ pub struct CrdtManager {
 
 #[allow(dead_code)]
 impl CrdtManager {
-    pub async fn new(base_dir: PathBuf) -> Result<Self, String> {
-        let sync_dir = base_dir.join(".sync");
+    pub async fn new(base_dir: PathBuf, config_dir: PathBuf) -> Result<Self, String> {
+        // Instead of writing to base_dir which causes "Read-only file system" errors
+        // on Android SAF URIs, we store sync states in the app's native config directory.
+        // We hash the base_dir path so multiple workspaces don't collide their sync states.
+
+        let workspace_hash = md5::compute(base_dir.to_string_lossy().as_bytes());
+        let sync_dir = config_dir.join(format!("sync_states_{:x}", workspace_hash));
+
         if !sync_dir.exists() {
             fs::create_dir_all(&sync_dir)
                 .await
-                .map_err(|e| format!("Failed to create .sync dir: {}", e))?;
+                .map_err(|e| format!("Failed to create sync dir: {}", e))?;
         }
 
         Ok(Self {

apps/app/src-tauri/src/sync/crdt.rs

@@ -40,7 +40,7 @@ impl SyncState {
         }
         path_str = urlencoding::decode(&path_str).unwrap_or(std::borrow::Cow::Borrowed(&path_str)).to_string();
 
-        let crdt_manager = CrdtManager::new(std::path::PathBuf::from(path_str)).await?;
+        let crdt_manager = CrdtManager::new(std::path::PathBuf::from(path_str), config_dir.clone()).await?;
 
         // Channel for server shutdown
         let (tx, _) = tokio::sync::broadcast::channel(1);