Fix who-read submenu hover crash

[redactdeveloper]

Summary

• guard who-read submenu population while the action is still loading or disabled
• defer auto-opening the submenu until the next event loop tick and re-check that the row is still selected/enabled

This avoids opening the read-receipts submenu from inside the async content update path, which matches the crash scenario described in #377 when hovering the who-read row while it is still loading.

Fixes #377.

Verification

• Ran git diff --check
• Full AyuGram Desktop build was not run locally because this checkout does not include the full dependency/build environment.