Chat: typed message is lost on failed send; first message vanishes in a new chat #1391

Split out of #1388 (items 1 and 2), verified against main (3959eb0).

Problem

Two ways a typed chat message disappears without a trace:

1. Failed send loses the question. The composer clears its input as soon as Send is pressed, and on error the hooks remove the optimistic message, so only a toast remains. Repro: DevTools → Network → Offline, send a message in notebook or source chat.
2. First message in a brand-new notebook chat vanishes while waiting. Creating the session enables the getSession query; it returns messages: [] while the LLM call is still running, and the sync effect overwrites the list that holds the optimistic message.

Root cause

• frontend/src/components/sources/ChatPanel.tsx:251-255 — handleSend calls onSendMessage(...) then setInput('') immediately (prop typed => void, line 38).
• frontend/src/lib/hooks/use-notebook-chat.ts:230-235 and use-source-chat.ts:201-206 — on error: setMessages(prev => prev.filter(msg => !msg.id.startsWith('temp-'))).
• use-notebook-chat.ts:59-63 — useEffect(() => { if (currentSession?.messages) setMessages(currentSession.messages) }) runs while a send is in flight. Same pattern in use-source-chat.ts:41-45.

Fix pointers

• Make sendMessage resolve to success/failure and onSendMessage return a Promise; restore the text into the input on failure (or keep the optimistic message marked as failed with a retry action).
• Skip the session→messages sync while isSending is true (or merge pending temp- messages into the fetched list).

Acceptance criteria

• With the network offline or a 5xx from the API, the typed text is still available to the user after the error toast (notebook chat and source chat).
• The first message of a new notebook chat stays visible until the reply arrives or fails.
• Hook tests cover both cases.