fix(tauri): add_workspace must initialise the target folder

The frontend currently calls init_workspace before add_workspace, but
the Tauri command itself is trivially breakable by any caller that
skips the pre-step or a future frontend refactor: add_workspace would
save the workspace entry pointing at a non-existent directory, and
every subsequent command would then fail with 'Path does not exist'
via TaskRepository::new. Call TaskRepository::init inside the command
so it is self-contained and idempotent.

apps/tauri/src-tauri/src/lib.rs

@@ -179,6 +179,13 @@ fn add_workspace(
     state: State<'_, Mutex<AppState>>,
 ) -> Result<(), String> {
     validate_workspace_path(&path)?;
+    // Ensure the path exists and is a valid workspace before persisting the
+    // config. Without this, calling add_workspace directly on a missing
+    // directory would save the workspace but every subsequent ensure_repo
+    // call would fail with "Path does not exist".
+    TaskRepository::init(PathBuf::from(&path))
+        .map(|_| ())
+        .map_err(|e| e.to_string())?;
     let mut s = lock_state(&state)?;
     let ws = WorkspaceConfig::new(name, PathBuf::from(&path));
     let id = s.config.add_workspace(ws);