fix: codechef submit fixes and atcoder cleanup (#355)

## Problem

After the initial CodeChef implementation, submit silently swallowed
"contest not available for submission" errors, and the submit flow used
blind `wait_for_timeout` delays. AtCoder had an unnecessary 500ms settle
delay after file upload.

## Solution

Replace the initial page-load sleep in CodeChef submit with
`wait_for_selector`, replace the 3s post-click sleep with a proper
`wait_for_selector` on the result dialog, and extend the practice
fallback
check to catch both dialog variants. Remove AtCoder's
`BROWSER_SETTLE_DELAY`
and the constant from `timeouts.py`.

scrapers/atcoder.py

@@ -30,7 +30,6 @@ from .timeouts import (
     BROWSER_ELEMENT_WAIT,
     BROWSER_NAV_TIMEOUT,
     BROWSER_SESSION_TIMEOUT,
-    BROWSER_SETTLE_DELAY,
     BROWSER_SUBMIT_NAV_TIMEOUT,
     BROWSER_TURNSTILE_POLL,
     HTTP_TIMEOUT,
@@ -500,7 +499,6 @@ def _submit_headless(
                     "buffer": Path(file_path).read_bytes(),
                 },
             )
-            page.wait_for_timeout(BROWSER_SETTLE_DELAY)
             page.locator('button[type="submit"]').click(no_wait_after=True)
             page.wait_for_url(
                 lambda url: "/submissions/me" in url,