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/submit.lua

@@ -110,10 +110,13 @@ function M.submit(opts)
             logger.log('Submitted successfully', { level = vim.log.levels.INFO, override = true })
           else
             local err = result and result.error or 'unknown error'
-            if err:match('^Login failed') then
+            if err == 'bad_credentials' or err:match('^Login failed') then
               cache.clear_credentials(platform)
             end
-            logger.log('Submit failed: ' .. err, { level = vim.log.levels.ERROR })
+            logger.log(
+              'Submit failed: ' .. (constants.LOGIN_ERRORS[err] or err),
+              { level = vim.log.levels.ERROR }
+            )
           end
         end)
       end