fix(login): remove cookie fast-path from login subcommand (#344)

## Problem

`:CP <platform> login` short-circuited on cached cookies/tokens — if an
old session was still valid, the new credentials were never tested. The
user got "login successful" even with wrong input. USACO submit also
wasted a roundtrip on `_check_usaco_login` every time.

## Solution

Always validate credentials in the login path. Remove cookie/token
loading from `_login_headless` (AtCoder), `_login_headless_cf` (CF),
`_login_headless_codechef` (CodeChef), and `login` (CSES). For USACO
submit, replace the `_check_usaco_login` roundtrip with cookie trust +
retry-on-auth-failure (the Kattis pattern). Submit paths are unchanged —
cookie fast-paths remain for contest speed.

Closes #331

scrapers/cses.py

@@ -239,21 +239,6 @@ class CSESScraper(BaseScraper):
             return self._login_error("Missing username or password")
 
         async with httpx.AsyncClient(follow_redirects=True) as client:
-            token = credentials.get("token")
-
-            if token:
-                print(json.dumps({"status": "checking_login"}), flush=True)
-                if await self._check_token(client, token):
-                    return LoginResult(
-                        success=True,
-                        error="",
-                        credentials={
-                            "username": username,
-                            "password": password,
-                            "token": token,
-                        },
-                    )
-
             print(json.dumps({"status": "logging_in"}), flush=True)
             token = await self._web_login(client, username, password)
             if not token: