fix(submit): harden atcoder and codeforces submit flow (#304)

## Problem

AtCoder file upload always wrote a `.cpp` temp file regardless of
language. CF submit used `solve_cloudflare=True` on the submit page,
causing a spurious "No Cloudflare challenge found" error;
`_wait_for_gate_reload` in `login_action` was dead code. Stale cookies
caused silent auth failures with no recovery path. The `uv.spawn` ndjson
path for submit had no overall timeout.

## Solution

Replace AtCoder's temp file with `page.set_input_files` using an
in-memory buffer and correct extension via `_LANGUAGE_ID_EXTENSION`.
Replace CF's temp-file/fallback dance with a direct
`textarea[name="source"]` fill and set `solve_cloudflare=False` on the
submit fetch. Add a login fast-path that skips the homepage check when
cookies exist, with automatic stale-cookie recovery via `_retried` flag
on redirect-to-login detection. Remove `_wait_for_gate_reload`. Fix
`_ensure_browser` to propagate install errors. Add a 120s kill timer to
the ndjson `uv.spawn` submit path in `scraper.lua`.

lua/cp/scraper.lua

@@ -85,6 +85,7 @@ local function run_scraper(platform, subcommand, args, opts)
     local stderr = uv.new_pipe(false)
     local buf = ''
 
+    local timer = nil
     local handle
     handle = uv.spawn(cmd[1], {
       args = vim.list_slice(cmd, 2),
@@ -92,6 +93,10 @@ local function run_scraper(platform, subcommand, args, opts)
       env = spawn_env_list(env),
       cwd = plugin_path,
     }, function(code, signal)
+      if timer and not timer:is_closing() then
+        timer:stop()
+        timer:close()
+      end
       if buf ~= '' and opts.on_event then
         local ok_tail, ev_tail = pcall(vim.json.decode, buf)
         if ok_tail then
@@ -124,6 +129,30 @@ local function run_scraper(platform, subcommand, args, opts)
       return { success = false, error = 'spawn failed' }
     end
 
+    if subcommand == 'submit' then
+      timer = uv.new_timer()
+      timer:start(120000, 0, function()
+        timer:stop()
+        timer:close()
+        if stdin_pipe and not stdin_pipe:is_closing() then
+          stdin_pipe:close()
+        end
+        if not stdout:is_closing() then
+          stdout:close()
+        end
+        if not stderr:is_closing() then
+          stderr:close()
+        end
+        if handle and not handle:is_closing() then
+          handle:kill(15)
+          handle:close()
+        end
+        if opts.on_exit then
+          opts.on_exit({ success = false, error = 'submit timed out' })
+        end
+      end)
+    end
+
     if stdin_pipe then
       uv.write(stdin_pipe, opts.stdin, function()
         uv.shutdown(stdin_pipe, function()