fix(auth): a Google connect names its tab's account too, and a refused tab says why

- /auth/google/connect is a navigation and carries no headers: the tab names the account it
  was built for in `expect`, checked by the same comparison as the request header
  (built_for_another_account); a mismatch lands on /?account=changed
- the link marker names the OAuth state of its own flow: an older flow coming back no longer
  spends the marker of the connect still open (which then returned as a plain sign-in)
- a 401 account_changed reloads the tab with a one-time flag, and the reloaded tab says it
  now shows the account signed in
- a refused switch to a suspended or not-yet-approved account says so in the signed-in
  app's words, naming the address a request was recorded for
- the prefs merge locks FOR NO KEY UPDATE, so a transaction that only added a row referring
  to the account does not hold the save up; a test pins the lock
- giving up a lost page closes a channel page open over it
- e2e README names the chromium-only specs

backend/tests/test_me_preferences.py

@@ -6,7 +6,8 @@ the request first read it — two merges over the same earlier read keep only th
 """
 
 from fastapi import Depends, Request
-from sqlalchemy import update
+from sqlalchemy import event, update
+from sqlalchemy.engine import Engine
 from sqlalchemy.orm import Session
 
 from app.auth import current_user
@@ -42,3 +43,32 @@ def test_a_save_keeps_a_key_another_save_stored_after_this_one_read_the_account(
     assert r.status_code == 200
     db.expire_all()
     assert db.get(User, user.id).preferences == {"seenVersion": "1.2.3", "startOnHome": False}
+
+
+def test_the_merge_reads_the_account_under_a_lock_that_orders_merges_and_nothing_else(
+    client, make_user
+):
+    # The re-read above is half of it: two merges that both read before either commits would still
+    # keep only the last, unless the read takes a lock the other merge waits for. FOR NO KEY UPDATE
+    # is that lock, and the weakest one: a plain FOR UPDATE would also wait behind any transaction
+    # that only added a row referring to the account (a playlist pull, then minutes of YouTube).
+    make_user(email="prefs-lock@example.com", password=_PW)
+    assert (
+        client.post(
+            "/auth/password-login", json={"email": "prefs-lock@example.com", "password": _PW}
+        )
+    ).status_code == 200
+    sent: list[str] = []
+
+    def record(_conn, _cursor, statement, *_rest):
+        sent.append(statement)
+
+    event.listen(Engine, "before_cursor_execute", record)
+    try:
+        assert client.put("/api/me/preferences", json={"startOnHome": False}).status_code == 200
+    finally:
+        event.remove(Engine, "before_cursor_execute", record)
+
+    locks = [s for s in sent if "FROM users" in s and " FOR " in s]
+    assert locks, sent
+    assert all(s.rstrip().endswith("FOR NO KEY UPDATE") for s in locks), locks