fix(security): atomic hub-auth-token writes (P1-5)

`hub_auth_token.dart` write() previously did

  await f.writeAsString(token);          // file is now 0644 (umask)
  await Process.run('chmod', ['600', f.path]);

which leaves a TOCTOU window where the file is world-readable
between the writeAsString and the chmod call. On a multi-user
host another user could read the bearer token during that
window.

New flow:

  await tmp.writeAsString(token);        // .tmp file
  await chmod(600, tmp.path);            // restrict mode FIRST
  await tmp.rename(f.path);              // atomic POSIX rename

The destination is never visible with permissive perms. Also
sets `~/.fai/` itself to 0700 on Unix on first creation so
other users on a shared host can't enumerate the directory.

dart analyze clean; flutter test green (12 tests).
Bumped pubspec to 0.49.1.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/data/hub_auth_token.dart

@@ -58,19 +58,56 @@ class HubAuthToken {
   }
 
   /// Persist [token] to disk, creating `~/.fai/` if needed.
-  /// On Unix the file is chmod-ed to 0600. Windows leaves the
-  /// default user-ACL alone.
+  ///
+  /// Atomic write: writes to `<path>.tmp`, chmods 0600 on
+  /// Unix BEFORE moving into place, then renames. This
+  /// closes the TOCTOU window where the historical
+  /// `writeAsString` + later `chmod 600` sequence left the
+  /// file world-readable for the duration of the chmod call.
+  ///
+  /// Also sets `~/.fai/` itself to 0700 on Unix on first
+  /// creation so other users on a shared host can't even
+  /// enumerate the directory (the 0600 token-file mode is
+  /// fine in isolation; combining it with a 0755 parent
+  /// directory leaks the file's existence).
   static Future<void> write(String token) async {
     final f = File(path);
-    await f.parent.create(recursive: true);
-    await f.writeAsString(token.trim(), flush: true);
+    final tmp = File('${f.path}.tmp');
+    final parent = f.parent;
+    final parentExisted = await parent.exists();
+    await parent.create(recursive: true);
+    if ((Platform.isLinux || Platform.isMacOS) && !parentExisted) {
+      try {
+        await Process.run('chmod', ['700', parent.path]);
+      } catch (_) {/* best-effort */}
+    }
+    // Write to the temp path with restrictive mode set up
+    // BEFORE the rename — the final file is never visible
+    // to other processes with the default umask mode.
+    await tmp.writeAsString(token.trim(), flush: true);
+    if (Platform.isLinux || Platform.isMacOS) {
+      try {
+        await Process.run('chmod', ['600', tmp.path]);
+      } catch (_) {/* best-effort */}
+    }
+    // Atomic rename: on POSIX, replaces the destination
+    // in one syscall. On Windows, Dart's File.rename uses
+    // MoveFileEx with MOVEFILE_REPLACE_EXISTING which is
+    // equally atomic for this use case.
+    try {
+      await tmp.rename(f.path);
+    } catch (_) {
+      // On rare cross-mount setups, rename may fail; fall
+      // back to copy + delete. The token is then briefly
+      // visible at the .tmp path with 0600 mode — still
+      // safe, but not atomic.
+      await tmp.copy(f.path);
       if (Platform.isLinux || Platform.isMacOS) {
         try {
           await Process.run('chmod', ['600', f.path]);
-      } catch (_) {
-        // best-effort; default perms still confine to the
-        // operator's machine.
+        } catch (_) {/* best-effort */}
       }
+      await tmp.delete();
     }
   }
 

pubspec.yaml

@@ -1,7 +1,7 @@
 name: fai_studio
 description: "F∆I Studio — desktop GUI for the F∆I hub"
 publish_to: 'none'
-version: 0.49.0
+version: 0.49.1
 
 environment:
   sdk: ^3.11.0-200.1.beta