fix: preserve first user message in new sessions

[SsparKluo]

Problem

In a newly created session, the first user message can disappear from the chat until the page is refreshed.

The race is between prompt_async and the initial getSessionMessages snapshot: the snapshot can be read before the user message is committed. The live message then gets overwritten by that stale snapshot.

The multi-server session-key migration also exposed a second issue: createSession() returns a raw session ID, while the message store and SSE pipeline use serverId::sessionId. The new-session send path used the raw ID for setStreaming and its fallback pull, so those writes went to a key the UI never reads.

Fix

• Scope the newly created session immediately with makeSessionKey(paneServerId, newSession.id).
• Use the scoped key for streaming state and fallback message/part updates.
• During an initial load, preserve local-only messages delivered by SSE when they are not older than the loaded snapshot.
• Keep older, out-of-window local messages discardable.

Verification

• npx vitest run — 670 tests passed
• npx tsc --noEmit — passed
• Added regression coverage for the new-session key scope and stale initial snapshot race.