fix(chat): prevent flicker from rapid 'Load older history' clicks - #42

Summary

Fixes chat flicker caused by rapidly clicking "Load older history" in the Electron renderer chat panel. Each click previously enqueued a serial read('older'); a burst churned the whole conversation history page-by-page, and each page triggered a full timeline re-render plus the scroll-anchoring layout effect, producing visible flickering.

The fix adds a per-pane in-flight guard in ConversationChatPanelForPane:

• loadOlder now returns early (no-op) when an older read is already pending or no older page exists, sets a synchronous loadingOlderRef, and clears it in a finally.
• The "Load older history" button is disabled and shows a "Loading older history…" label while a load is in flight, then re-enables on completion.

Validation

• npm run verify passes locally (typecheck, biome lint, 584 vitest tests, site:build).
• src/renderer/chat/ConversationChatPanel.test.tsx adds regression coverage:
  • rapid clicks dedupe to a single in-flight older read,
  • button disabled + loading label while pending, re-enabled after resolution,
  • subsequent single clicks still work and use the updated olderCursor.
• Manual reproduction steps: with the chat panel open, scroll to the top, throttle the engine read, click "Load older history" 20+ times within ~1s — only one older read executes and no page-jump flicker occurs.

Checklist

• The change is focused and does not include unrelated work.
• Tests cover the new behavior.
• npm run verify passes locally.
• No credentials, tokens, private data, or generated build artifacts are included.