fix(submit): use file path over stdin; fix CF CodeMirror textarea (#305)

## Problem

After the initial submit hardening, two issues remained: source code was
read in Lua and piped as stdin to the scraper (unnecessary roundtrip
since
the file exists on disk), and CF's `page.fill()` timed out on the hidden
`textarea[name="source"]` because CodeMirror owns the editor state.

## Solution

Pass the source file path as a CLI arg instead — AtCoder calls
`page.set_input_files(file_path)` directly, CF reads it with
`Path(file_path).read_text()`. Fix CF source injection via
`page.evaluate()`
into the CodeMirror instance. Extract `BROWSER_SUBMIT_NAV_TIMEOUT` as a
per-platform `defaultdict` (CF defaults to 2× nav timeout). Save the
buffer
with `vim.cmd.update()` before submitting.

lua/cp/log.lua

@@ -1,12 +1,27 @@
 local M = {}
 
-function M.log(msg, level, override)
+---@class LogOpts
+---@field level? integer
+---@field override? boolean
+---@field sync? boolean
+
+---@param msg string
+---@param opts? LogOpts
+function M.log(msg, opts)
   local debug = require('cp.config').get_config().debug or false
-  level = level or vim.log.levels.INFO
+  opts = opts or {}
+  local level = opts.level or vim.log.levels.INFO
+  local override = opts.override or false
+  local sync = opts.sync or false
   if level >= vim.log.levels.WARN or override or debug then
-    vim.schedule(function()
+    local notify = function()
       vim.notify(('[cp.nvim]: %s'):format(msg), level)
-    end)
+    end
+    if sync then
+      notify()
+    else
+      vim.schedule(notify)
+    end
   end
 end