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/submit.lua

@@ -53,9 +53,7 @@ function M.submit(opts)
   end
 
   prompt_credentials(platform, function(creds)
-    local source_lines = vim.fn.readfile(source_file)
-    local source_code = table.concat(source_lines, '\n')
-
+    vim.cmd.update()
     vim.notify('[cp.nvim] Submitting...', vim.log.levels.INFO)
 
     require('cp.scraper').submit(
@@ -63,7 +61,7 @@ function M.submit(opts)
       contest_id,
       problem_id,
       language,
-      source_code,
+      source_file,
       creds,
       function(ev)
         vim.schedule(function()