Conversation view shows 'How can I help you' empty-state instead of a loading skeleton while the transcript loads

[eltmon]

PRD: https://github.com/eltmon/overdeck/blob/overdeck-state/drafts/pan-3744.md

When opening a conversation, the panel renders the new-conversation greeting until the transcript RPC resolves (which can take many seconds under load — see the queueing issue). The greeting is an assertion that the conversation is empty; while loading, that is wrong information.

Work: distinguish 'transcript not yet loaded' from 'conversation genuinely empty' and render a skeleton for the former.

AC: with a throttled/slow transcript response, opening a conversation shows a skeleton, never the greeting, until messages arrive.

[eltmon]

🤖 Agent completed work:

Anomalies: the full repository suite passed 13,864 tests but 28 unrelated tests timed out at the shared 5-second limit during concurrent database setup. The first sandboxed full-test attempt also hit spawnSync git EPERM. Plan deviations: the test-conv non-streaming assumption was stale, so the WebSocket mock now preserves explicit initial payloads; the skeleton component was extracted to satisfy the file-size guard. Delivered first-payload gating, transcript skeleton with reduced-motion support, delayed-stream and switch tests, and loading-state docs. Feature-targeted tests, typecheck, and lint pass; the branch is clean and fully pushed.