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/codeforces.py

@@ -8,7 +8,13 @@ from typing import Any
 import requests
 from bs4 import BeautifulSoup, Tag
 
-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 .models import (
     ContestListResult,
     ContestSummary,
@@ -387,7 +393,9 @@ def _login_headless_cf(credentials: dict[str, str]) -> LoginResult:
                 google_search=False,
                 cookies=saved_cookies,
             ) as session:
-                session.fetch(f"{BASE_URL}/", page_action=check_action, solve_cloudflare=True)
+                session.fetch(
+                    f"{BASE_URL}/", page_action=check_action, solve_cloudflare=True
+                )
             if logged_in:
                 return LoginResult(success=True, error="")
         except Exception:
@@ -419,7 +427,9 @@ def _login_headless_cf(credentials: dict[str, str]) -> LoginResult:
 
             session.fetch(f"{BASE_URL}/", page_action=verify_action, network_idle=True)
             if not logged_in:
-                return LoginResult(success=False, error="Login failed (bad credentials?)")
+                return LoginResult(
+                    success=False, error="Login failed (bad credentials?)"
+                )
 
             try:
                 browser_cookies = session.context.cookies()
@@ -445,7 +455,6 @@ def _submit_headless(
 
     source_code = Path(file_path).read_text()
 
-
     try:
         from scrapling.fetchers import StealthySession  # type: ignore[import-untyped,unresolved-import]
     except ImportError:
@@ -519,7 +528,9 @@ def _submit_headless(
         ) as session:
             if not _retried and saved_cookies:
                 print(json.dumps({"status": "checking_login"}), flush=True)
-                session.fetch(f"{BASE_URL}/", page_action=check_login, solve_cloudflare=True)
+                session.fetch(
+                    f"{BASE_URL}/", page_action=check_login, solve_cloudflare=True
+                )
 
             if not logged_in:
                 print(json.dumps({"status": "logging_in"}), flush=True)