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".

frontend/src/main.tsx

@@ -77,7 +77,8 @@ const root =
   ) : (
     <QueryClientProvider client={queryClient}>
       {/* One tree for the whole app: every anchored popover registers as a node so Escape and
-          outside-press act only on the topmost layer (the Modal and the players join it too). */}
+          outside-press act only on the topmost popover. The Modal and the players are not nodes:
+          they order against the popovers through lib/layerStack. */}
       <FloatingTree>
         <ConfirmProvider>
           {/* Nothing below renders until the tab knows which account it is: every provider