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/pickers/init.lua

@@ -42,10 +42,10 @@ function M.get_platform_contests(platform, refresh)
   local picker_contests = cache.get_contest_summaries(platform)
 
   if refresh or vim.tbl_isempty(picker_contests) then
+    local display_name = constants.PLATFORM_DISPLAY_NAMES[platform]
     logger.log(
-      ('Loading %s contests...'):format(constants.PLATFORM_DISPLAY_NAMES[platform]),
-      vim.log.levels.INFO,
-      true
+      ('Fetching %s contests...'):format(display_name),
+      { level = vim.log.levels.INFO, override = true, sync = true }
     )
 
     local contests = scraper.scrape_contest_list(platform)
@@ -53,12 +53,8 @@ function M.get_platform_contests(platform, refresh)
     picker_contests = cache.get_contest_summaries(platform)
 
     logger.log(
-      ('Loaded %d %s contests.'):format(
-        #picker_contests,
-        constants.PLATFORM_DISPLAY_NAMES[platform]
-      ),
-      vim.log.levels.INFO,
-      true
+      ('Fetched %d %s contests.'):format(#picker_contests, display_name),
+      { level = vim.log.levels.INFO, override = true }
     )
   end