Sub-thread icons no longer render broken

Every thread gets its own generated avatar, and in the chat that avatar was being shown the wrong way: the sub-thread chip inside a message and the "Running sub-threads" card below the conversation both drew the browser's broken-image placeholder instead of the picture.

They now show the thread's real avatar, and any icon the portal cannot draw — an unknown icon name, an empty one — falls back to the standard chat icon rather than a broken image. The same fix applies to the model and agent pickers in the chat composer, where icons of that kind used to be dropped silently.