fix: keep chart visible during refetch to avoid flicker (Closes #1411)

[waterWang]

Summary

Fixes the chart flicker on the Financial Insights page (#1411).

Before: every period change / retry set loading = true, which replaced the chart with a skeleton, then re-rendered the chart when data arrived — a visible "skeleton flash" on every refetch.

After: the already-rendered chart stays visible while refetching. The skeleton is only shown on the very first load, and a failed refetch keeps the previous data on screen instead of showing an error state over existing content.

Changes

• app/dashboard/insight/page.tsx
  • Added a hasData flag tracking whether any data has loaded successfully.
  • Only set loading = true on the first load (when !hasData).
  • On a failed refetch, keep the previous data visible and suppress the error state when stale data already exists.
  • Render the skeleton only when loading && !hasData.

Tests

Added tests/react/insight-page-flicker.test.tsx covering:

• Initial load shows skeleton, then the chart after data arrives.
• Period change keeps the chart visible (no skeleton flash) while refetching.
• Initial load failure shows the error state.
• Refetch failure after initial success keeps the previous data and does not show the error state.

All 4 tests pass.

Closes #1411