fix(scrapers): bad credentials detection and error message cleanup

Problem: Wrong credentials during login produced two bugs: scrapers
wrapped the `bad_credentials` error code in `"Login failed: ..."`,
causing double-prefixed messages in the UI; and `credentials.lua`
did not clear cached credentials before re-prompting or on failure,
leaving stale bad creds in the cache.

Solution: Standardize all scrapers to emit `"bad_credentials"` as
the raw error code. Add `LOGIN_ERRORS` map in `constants.lua` to
translate it to a human-readable string in both `credentials.lua`
and `submit.lua`. Fix `credentials.lua` to clear credentials on
failure in both the fresh-prompt and cached-creds-fail paths.
For AtCoder and Codeforces, replace `wait_for_url` with
`wait_for_function` to detect the login error element immediately
rather than waiting the full 10s navigation timeout. Also add
"Remember me" checkbox check on Codeforces login.

lua/cp/constants.lua

@@ -221,4 +221,8 @@ M.DEFAULT_VERSIONS = { cpp = 'c++20', python = 'python3' }
 
 M.COOKIE_FILE = vim.fn.expand('~/.cache/cp-nvim/cookies.json')
 
+M.LOGIN_ERRORS = {
+  bad_credentials = 'bad credentials',
+}
+
 return M