fix(credentials): try cached credentials before prompting on login

Problem: `:CP <platform> login` always prompted for username and
password, even when valid credentials were already cached.

Solution: if cached credentials exist, attempt `scraper.login()`
with them first. On success, return immediately with no prompt. On
failure, fall through to `prompt_and_login()` for fresh input.

lua/cp/credentials.lua

@@ -11,6 +11,43 @@ local STATUS_MESSAGES = {
   installing_browser = 'Installing browser...',
 }
 
+---@param platform string
+---@param display string
+local function prompt_and_login(platform, display)
+  vim.ui.input({ prompt = 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: ')
+    vim.fn.inputrestore()
+    if not password or password == '' then
+      logger.log('Cancelled', { level = vim.log.levels.WARN })
+      return
+    end
+
+    local credentials = { username = username, password = password }
+
+    local scraper = require('cp.scraper')
+    scraper.login(platform, credentials, function(ev)
+      vim.schedule(function()
+        local msg = STATUS_MESSAGES[ev.status] or ev.status
+        logger.log(display .. ': ' .. msg, { level = vim.log.levels.INFO, override = true })
+      end)
+    end, function(result)
+      vim.schedule(function()
+        if result.success then
+          logger.log(display .. ' login successful', { level = vim.log.levels.INFO, override = true })
+        else
+          local err = result.error or 'unknown error'
+          logger.log(display .. ' login failed: ' .. err, { level = vim.log.levels.ERROR })
+        end
+      end)
+    end)
+  end)
+end
+
 ---@param platform string?
 function M.login(platform)
   platform = platform or state.get_platform()
@@ -24,31 +61,12 @@ function M.login(platform)
 
   local display = constants.PLATFORM_DISPLAY_NAMES[platform] or platform
 
-  vim.ui.input({ prompt = 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: ')
-    vim.fn.inputrestore()
-    if not password or password == '' then
-      logger.log('Cancelled', { level = vim.log.levels.WARN })
-      return
-    end
-
   cache.load()
   local existing = cache.get_credentials(platform) or {}
-    local credentials = {
-      username = username,
-      password = password,
-    }
-    if existing.token then
-      credentials.token = existing.token
-    end
 
+  if existing.username and existing.password then
     local scraper = require('cp.scraper')
-    scraper.login(platform, credentials, function(ev)
+    scraper.login(platform, existing, function(ev)
       vim.schedule(function()
         local msg = STATUS_MESSAGES[ev.status] or ev.status
         logger.log(display .. ': ' .. msg, { level = vim.log.levels.INFO, override = true })
@@ -56,17 +74,16 @@ function M.login(platform)
     end, function(result)
       vim.schedule(function()
         if result.success then
-          logger.log(
+          logger.log(display .. ' login successful', { level = vim.log.levels.INFO, override = true })
-            display .. ' login successful',
-            { level = vim.log.levels.INFO, override = true }
-          )
         else
-          local err = result.error or 'unknown error'
+          prompt_and_login(platform, display)
-          logger.log(display .. ' login failed: ' .. err, { level = vim.log.levels.ERROR })
         end
       end)
     end)
-  end)
+    return
+  end
+
+  prompt_and_login(platform, display)
 end
 
 ---@param platform string?