fix: warn when uploaded cookies shadow a configured cookiefile

Reported in #881, where the reporter had to reverse-engineer this from
the outside over several days: setting
YTDL_OPTIONS={"cookiefile": "/cookies/cookies.txt"} appeared to do
nothing whenever a cookies file had also been uploaded through the UI.

Uploaded cookies winning is correct and stays as it is. The upload
exists so cookies can be refreshed without restarting the container, and
letting YTDL_OPTIONS win instead would leave a visible UI button that
silently does nothing.

The defect is that it happened in silence, and could not be reported
afterwards even in principle. set_runtime_override writes into
YTDL_OPTIONS directly, so the moment an uploaded file is applied the
configured path is gone from the live config: delete_cookies' existing
has_manual_cookiefile check compares against COOKIES_PATH and therefore
cannot fire once the value has been replaced. The two override points
are the only places where both paths are still visible, so that is where
the warning has to go.

The startup path previously logged only "Cookie file detected"; both it
and the upload handler now say plainly which file is being ignored and
how to get it back.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/tests/test_main_helpers.py

@@ -313,3 +313,40 @@ class GetCustomDirsTests(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+class WarnIfCookiefileShadowedTests(unittest.TestCase):
+    """Issue #881: an uploaded cookies file wins over an operator-configured
+    cookiefile, and used to do so with no way for anyone to notice."""
+
+    def setUp(self):
+        self._saved = main.config.YTDL_OPTIONS
+        main.config.YTDL_OPTIONS = dict(self._saved)
+
+    def tearDown(self):
+        main.config.YTDL_OPTIONS = self._saved
+
+    def test_warns_when_a_different_cookiefile_is_configured(self):
+        main.config.YTDL_OPTIONS["cookiefile"] = "/cookies/cookies.txt"
+        with self.assertLogs("main", level="WARNING") as cm:
+            main.warn_if_cookiefile_shadowed()
+        joined = "\n".join(cm.output)
+        self.assertIn("/cookies/cookies.txt", joined)
+        self.assertIn(main.COOKIES_PATH, joined)
+
+    def test_silent_when_no_cookiefile_configured(self):
+        main.config.YTDL_OPTIONS.pop("cookiefile", None)
+        with self.assertNoLogs("main", level="WARNING"):
+            main.warn_if_cookiefile_shadowed()
+
+    def test_silent_when_configured_file_is_the_uploaded_one(self):
+        # The steady state after an upload: re-running must not nag.
+        main.config.YTDL_OPTIONS["cookiefile"] = main.COOKIES_PATH
+        with self.assertNoLogs("main", level="WARNING"):
+            main.warn_if_cookiefile_shadowed()
+
+    def test_silent_on_non_string_or_empty_values(self):
+        for value in (None, "", 0, [], {}):
+            main.config.YTDL_OPTIONS["cookiefile"] = value
+            with self.assertNoLogs("main", level="WARNING"):
+                main.warn_if_cookiefile_shadowed()