docs(comments): correct comments the cuts left stale

The registration language is read today (preferences.language, the mail language); the Modal and
the players order against popovers through layerStack, not the FloatingTree; the upload's auth
dependency is current_user; the module of access tests covers onboarding and access, not only the
canonical email. The dev proxy's WebSocket upgrade served the removed messages socket and goes; the
test compose header says a pure-logic run needs SIFTLODE_TEST_REQUIRE_DB=0; the speed measure no
longer reports an unreachable database as "no share link".

backend/app/routes/auth.py

@@ -298,8 +298,9 @@ class EmailPasswordIn(BaseModel):
     # The language the sign-up page was in. Captured at registration because it is the ONLY moment
     # the choice exists anywhere the server can see: it lives in the browser until an account does,
     # and a preference nobody recorded cannot be backfilled later (as `google_email` showed: those
-    # rows still read NULL until each user signs in again). Nothing reads it yet; the bilingual
-    # mail templates will.
+    # rows still read NULL until each user signs in again). It is stored as the account's
+    # `preferences["language"]`, which the UI starts in and every mail to the account is written in
+    # (`mailtext.lang_for_user` / `lang_for_email`).
     lang: str | None = None