fix(scrapers): bad credentials detection and error message cleanup (#367)

## Problem

Wrong credentials produced garbled error messages (`"login failed: Login
failed: bad_credentials"`) and stale credentials remained cached after
failure, causing silent re-use on the next invocation.

## Solution

Standardize all scrapers to emit `"bad_credentials"` as a plain error
code, mapped to a human-readable string via `LOGIN_ERRORS` in
`constants.lua`. Fix `credentials.lua` to clear cached credentials on
failure in both the fresh-prompt and re-prompt paths. For AtCoder and
Codeforces, replace `wait_for_url` with `wait_for_function` to detect
the login error element immediately rather than sitting the full 10s
navigation timeout. Add "Remember me" checkbox 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

@@ -14,13 +14,13 @@ local STATUS_MESSAGES = {
 ---@param platform string
 ---@param display string
 local function prompt_and_login(platform, display)
-  vim.ui.input({ prompt = display .. ' username: ' }, function(username)
+  vim.ui.input({ prompt = '[cp.nvim]: ' .. display .. ' username: ' }, function(username)
     if not username or username == '' then
       logger.log('Cancelled', { level = vim.log.levels.WARN })
       return
     end
     vim.fn.inputsave()
-    local password = vim.fn.inputsecret(display .. ' password: ')
+    local password = vim.fn.inputsecret('[cp.nvim]: ' .. display .. ' password: ')
     vim.fn.inputrestore()
     if not password or password == '' then
       logger.log('Cancelled', { level = vim.log.levels.WARN })
@@ -45,7 +45,11 @@ 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 +87,7 @@ 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