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.

scrapers/usaco.py

@@ -73,8 +73,11 @@ def _parse_results_page(html: str) -> dict[str, list[tuple[str, str]]]:
     for part in parts:
         heading_m = DIVISION_HEADING_RE.search(part)
         if heading_m:
-            current_div = heading_m.group(3).lower()
-            sections.setdefault(current_div, [])
+            div = heading_m.group(3)
+            if div:
+                key = div.lower()
+                current_div = key
+                sections.setdefault(key, [])
             continue
         if current_div is not None:
             for m in PROBLEM_BLOCK_RE.finditer(part):
@@ -285,7 +288,7 @@ class USACOScraper(BaseScraper):
         self,
         contest_id: str,
         problem_id: str,
-        source_code: str,
+        file_path: str,
         language_id: str,
         credentials: dict[str, str],
     ) -> SubmitResult: