[D-26] Empty assistant message persists when stream is aborted before first token

[rz1989s]

Discovered during Sprint 4.2b Chrome MCP verification (PR for #23 + #24).

Issue

When the user submits a query, useChat.ts:131-150 pushes BOTH the user message AND an empty assistant message into the conversation array BEFORE streaming starts. If the stream is then aborted (via the in-input stop button, or via switchConversation/deleteConversation), the empty assistant message stays in the conversation array forever.

Visible in production: opening any old chat shows orphan K avatars with no content beside them — one per aborted stream.

Repro

1. Open https://kami.rectorspace.com
2. Submit any query
3. Click the stop button BEFORE the first token arrives (~2s window)
4. Observe: the user message stays visible; an orphan K avatar persists alongside it
5. Switch to another conversation and back — the orphan stays

Suggested fix (needs brainstorm)

Two reasonable approaches, neither obviously correct without more thought:

• A: Delete the empty assistant message on abort. Simple but loses the breadcrumb that "the user asked X but it was interrupted."
• B: Replace the empty content with a placeholder string (e.g., "Stream interrupted.") so the row remains visually meaningful. Slightly more work; preserves history.

Files

• src/hooks/useChat.ts:131-150 (push empty assistant message)
• src/hooks/useChat.ts:86-90 (switchConversation aborts)
• src/hooks/useChat.ts:100-118 (deleteConversation aborts)
• src/hooks/useChat.ts:300-303 (stopStreaming)

Effort

~1-2h once the brainstorm picks an approach.

Tracked by

Not part of the 2026-04-26 QA umbrella (#3) — discovered later. Stand-alone issue.