FileExistsError on Windows when writing history backup file #41

`write_json_file` in `core/init.py` (called from `persist_requests`, which every successful request triggers) writes history atomically via a temp file renamed to a `.bkp` backup path:

```
File "core/init.py", line 407, in write_json_file
FileExistsError: [WinError 183] Cannot create a file when that file already exists: '...\Packages\User\Requester.history.json' -> '...\Packages\User\Requester.history.json.bkp'
```

`os.rename()` on POSIX silently overwrites an existing destination; on Windows it raises `FileExistsError` instead if the destination already exists. Since `Requester.history.json.bkp` already existed on this machine from a prior run, the very next request crashed here.

Reproduction: run any request once (creating `Requester.history.json.bkp`), then run a second request -- the second one hits the `.bkp` rename and raises.

The exception doesn't visibly break the visible request/response flow (the response tab still renders correctly), so it's easy to miss unless the console is checked -- this was found via a full console read, not a failed request.

Suggested fix: use `os.replace()` instead of `os.rename()` for the backup step -- `os.replace()` is documented to overwrite an existing destination atomically on both POSIX and Windows.

Sublime Text 4200, Windows.