fix(settings): handle empty or corrupted windows.json without crashing

[Wasdalt]

Problem

If ~/.config/forge/config/windows.json exists but is empty (0 bytes) or contains invalid JSON, JSON.parse throws a SyntaxError: unexpected end of data at line 1 column 1, causing the GNOME extension to fail initialization and enter ERROR state.

In addition, loadFile did not close the file stream when creating the default config, which could lead to empty files being created.

Solution

1. In get windowProps(), wrap JSON.parse in a try...catch block and verify that windowConfigContents is a non-empty string. Fall back to loadDefaultWindowConfigContents() if parsing fails.
2. In loadFile(), ensure the created stream is closed via close() and return the newly created file descriptor instead of null