feat: file conflict prompt, empty submit guard, and lint fixes (#366)

## Problem

Loading a problem whose source file already exists silently overwrites
user code. Submitting an empty file sends a blank submission to the
platform. Two ruff lint violations existed in the scrapers.

## Solution

- `setup.lua`: when the target source file exists on the filesystem
(`vim.uv.fs_stat`), show an inline `Overwrite? [y/N]:` prompt. Declining
keeps the existing file open and registers state normally. Skipped when
the file is already loaded in a buffer.
- `submit.lua`: resolve path to absolute, use `vim.uv.fs_stat` to verify
existence, abort with WARN if `stat.size == 0` ("Submit aborted: source
file has no content").
- `scrapers/atcoder.py`: remove unused `pathlib.Path` import (F401).
- `scrapers/base.py`: move local imports to top of file (E402).

Closes #364, #365.

scrapers/kattis.py

@@ -10,7 +10,13 @@ from pathlib import Path
 
 import httpx
 
-from .base import BaseScraper, clear_platform_cookies, extract_precision, load_platform_cookies, save_platform_cookies
+from .base import (
+    BaseScraper,
+    clear_platform_cookies,
+    extract_precision,
+    load_platform_cookies,
+    save_platform_cookies,
+)
 from .timeouts import HTTP_TIMEOUT
 from .models import (
     ContestListResult,