fix(scrapers): login fast paths and re-auth hardening for httpx platforms (#357)

## Problem

On CSES, Kattis, and USACO, `:CP <platform> login` always prompted
for credentials and ran a full web login even when a valid session was
already cached. Submit also had weak stale-session detection.

## Solution

`credentials.lua` now tries cached credentials first before prompting,
delegating fast-path detection to each scraper. CSES `login()` checks
the cached API token and returns immediately if valid. USACO `login()`
and `submit()` call `_check_usaco_login()` upfront. Kattis `submit()`
emits `checking_login` consistently and also triggers re-auth on HTTP
400/403, not just on the `"Request validation failed"` text match.
The premature `Submitting...` log emitted by Lua before the scraper
started is removed — Python's own status events are sufficient.

lua/cp/credentials.lua

@@ -11,19 +11,9 @@ local STATUS_MESSAGES = {
   installing_browser = 'Installing browser...',
 }
 
----@param platform string?
-function M.login(platform)
-  platform = platform or state.get_platform()
-  if not platform then
-    logger.log(
-      'No platform specified. Usage: :CP <platform> login',
-      { level = vim.log.levels.ERROR }
-    )
-    return
-  end
-
-  local display = constants.PLATFORM_DISPLAY_NAMES[platform] or platform
-
+---@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 })
@@ -37,15 +27,7 @@ function M.login(platform)
       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
+    local credentials = { username = username, password = password }
 
     local scraper = require('cp.scraper')
     scraper.login(platform, credentials, function(ev)
@@ -69,6 +51,47 @@ function M.login(platform)
   end)
 end
 
+---@param platform string?
+function M.login(platform)
+  platform = platform or state.get_platform()
+  if not platform then
+    logger.log(
+      'No platform specified. Usage: :CP <platform> login',
+      { level = vim.log.levels.ERROR }
+    )
+    return
+  end
+
+  local display = constants.PLATFORM_DISPLAY_NAMES[platform] or platform
+
+  cache.load()
+  local existing = cache.get_credentials(platform) or {}
+
+  if existing.username and existing.password then
+    local scraper = require('cp.scraper')
+    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 })
+      end)
+    end, function(result)
+      vim.schedule(function()
+        if result.success then
+          logger.log(
+            display .. ' login successful',
+            { level = vim.log.levels.INFO, override = true }
+          )
+        else
+          prompt_and_login(platform, display)
+        end
+      end)
+    end)
+    return
+  end
+
+  prompt_and_login(platform, display)
+end
+
 ---@param platform string?
 function M.logout(platform)
   platform = platform or state.get_platform()

lua/cp/submit.lua

@@ -79,7 +79,6 @@ function M.submit(opts)
 
   prompt_credentials(platform, function(creds)
     vim.cmd.update()
-    logger.log('Submitting...', { level = vim.log.levels.INFO, override = true })
 
     require('cp.scraper').submit(
       platform,