fix: resolve race between the chat greeting and history restore

[cpsievert]

Problem

When both a greeting and chat history are enabled, the greeting could show at the wrong time.

• Reloading a page that restores an existing conversation would briefly flash the app's greeting before the restored transcript replaced it.
• Starting a new chat in a session that began by restoring an existing conversation would never show a greeting again for the rest of that session.

To reproduce (before this fix):

Chat(
    id="chat",
    client=my_client,
    greeting=chat_greeting("## Welcome!", persistent=True),
    history=HistoryOptions(...),
)

1. Load the app, send a message (this saves a conversation).
2. Reload the page — the conversation restores, but the greeting flashes briefly first.
3. Open the history drawer and click "New chat" — no greeting ever appears again this session.

Root cause

The greeting used to resolve via an independent client-side signal ({id}_greeting_requested, which fires whenever the chat is visible, empty, and has no greeting) that raced the server's async history-restore decision. Whichever finished first determined the (inconsistent) outcome.

Fix

Greeting resolution now defers to the same "is this conversation restored?" decision HistoryController already computes — once at startup, and now also on every new_chat() — instead of racing the client's independent request:

• HistoryController.replay_ui() (used both on initial restore and when switching conversations from the drawer) now explicitly clears any greeting, since a restored conversation is never a "new chat".
• HistoryController.new_chat() now re-resolves the greeting directly, so a fresh conversation always gets one — even in a session that began by restoring an existing conversation.

Applied symmetrically in pkg-py and pkg-r.

Testing

• New Playwright e2e coverage: no greeting flash on restore, greeting clears when switching to an old conversation, and greeting reappears after "New chat" in a session that started by restoring a conversation.
• Unit tests for the new HistoryController.on_settled/notify_settled hook in both languages.
• Existing greeting e2e tests were switched to use persistent=True greetings — a plain-string greeting is auto-dismissed by the client regardless of this fix, so those tests passed even against the buggy code (a false negative).