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

lua/cp/credentials.lua

@@ -11,6 +11,7 @@ local STATUS_MESSAGES = {
   installing_browser = 'Installing browser...',
 }
 
+
 ---@param platform string
 ---@param display string
 local function prompt_and_login(platform, display)
@@ -45,7 +46,11 @@ local function prompt_and_login(platform, display)
           )
         else
           local err = result.error or 'unknown error'
-          logger.log(display .. ' login failed: ' .. err, { level = vim.log.levels.ERROR })
+          cache.clear_credentials(platform)
+          logger.log(
+            display .. ' login failed: ' .. (constants.LOGIN_ERRORS[err] or err),
+            { level = vim.log.levels.ERROR }
+          )
         end
       end)
     end)
@@ -83,6 +88,7 @@ function M.login(platform)
             { level = vim.log.levels.INFO, override = true }
           )
         else
+          cache.clear_credentials(platform)
           prompt_and_login(platform, display)
         end
       end)

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