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/restore.lua

@@ -11,7 +11,7 @@ function M.restore_from_current_file()
   local current_file = (vim.uv.fs_realpath(vim.fn.expand('%:p')) or vim.fn.expand('%:p'))
   local file_state = cache.get_file_state(current_file)
   if not file_state then
-    logger.log('No cached state found for current file.', vim.log.levels.ERROR)
+    logger.log('No cached state found for current file.', { level = vim.log.levels.ERROR })
     return false
   end