refactor(submit): pass file path instead of source via stdin

Problem: Submit read the source file in Lua, piped the full content as
stdin through to Python, then re-encoded it into an in-memory buffer
just to hand it back to the browser's file input. Unnecessary roundtrip
for AtCoder; CF also gains nothing from Lua owning the read.

Solution: Pass `source_file` path as a CLI arg to the scraper instead
of reading it in Lua and streaming via stdin. AtCoder calls
`page.set_input_files(file_path)` directly. Codeforces reads the file
with `Path(file_path).read_text()` before the browser session. Also
saves the buffer with `vim.cmd.update()` before submitting.

lua/cp/scraper.lua

@@ -316,15 +316,14 @@ function M.submit(
   contest_id,
   problem_id,
   language,
-  source_code,
+  source_file,
   credentials,
   on_status,
   callback
 )
   local done = false
-  run_scraper(platform, 'submit', { contest_id, problem_id, language }, {
+  run_scraper(platform, 'submit', { contest_id, problem_id, language, source_file }, {
     ndjson = true,
-    stdin = source_code,
     env_extra = { CP_CREDENTIALS = vim.json.encode(credentials) },
     on_event = function(ev)
       if ev.credentials ~= nil then