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/base.py

@@ -7,6 +7,16 @@ from abc import ABC, abstractmethod
 from pathlib import Path
 from typing import Any
 
+from .language_ids import get_language_id
+from .models import (
+    CombinedTest,
+    ContestListResult,
+    LoginResult,
+    MetadataResult,
+    SubmitResult,
+    TestsResult,
+)
+
 _COOKIE_FILE = Path.home() / ".cache" / "cp-nvim" / "cookies.json"
 
 
@@ -37,16 +47,6 @@ def clear_platform_cookies(platform: str) -> None:
         pass
 
 
-from .language_ids import get_language_id
-from .models import (
-    CombinedTest,
-    ContestListResult,
-    LoginResult,
-    MetadataResult,
-    SubmitResult,
-    TestsResult,
-)
-
 _PRECISION_ABS_REL_RE = re.compile(
     r"(?:absolute|relative)\s+error[^.]*?10\s*[\^{]\s*\{?\s*[-\u2212]\s*(\d+)\s*\}?",
     re.IGNORECASE,