fix: show loading/error state on the velocity heatmap page

[jonsuguiyama]

🤖 This PR was written with the help of an AI coding agent (Claude), reviewed and submitted by @jonsuguiyama.

Closes #1483.

Root cause

useVelocityAggregationData already returns loading/error from react-query,
and VelocityHeatmapRectangles rendered a <div className="err"> for it, but
that div is a child of react-leaflet's MapContainer, which only knows how to
position actual map layers (Rectangle, Popup, TileLayer, etc.). A plain
div there never gets a usable position, so it stays invisible in practice even
though the code appears to handle loading/error.

Fix

• VelocityHeatmapRectangles now reports status via a setStatus?(loading, hasError)
  callback prop, mirroring the existing setMinMax pattern already used for the legend.
• VelocityHeatmapPage owns the loading/error state and renders it as a normal
  MUI Box/CircularProgress overlay positioned above MapShell, outside the
  Leaflet children tree.
• Background and text colors are theme-aware (theme.palette.mode), matched to
  the exact light/dark values the map legend already uses ($legend-bg/
  $legend-bg-dark in map.scss), so the text has real contrast in both themes
  instead of defaulting to the app's text color against a fixed light background.
• Removed the now-unused .err style and its now-empty .scss file.

Note

While testing locally, the production siri_velocity_aggregation API endpoint
hung (timed out) for every date/bounds combination tried, while the rest of the
API responded normally. That looks like a separate backend issue, out of scope
for this fix.

Test plan

• npm run lint (tsc + eslint + stylelint + prettier), clean
• npx tsc && npx vite build, builds successfully
• npx vitest run src/pages/velocityHeatmap, 7/7 existing tests pass

[github-actions]

Preview: https://s3.amazonaws.com/noam-gaash.co.il/31753715526/open-bus/3074/index.html
Preview Storybook: https://s3.amazonaws.com/noam-gaash.co.il/31753715526/open-bus/3074/storybook/index.html
Test Report (if available): 'https://s3.amazonaws.com/noam-gaash.co.il/31753715526/open-bus/a7551a011638d54bb0a2608177c6a194e8a1a794/test-results/index.html'

[NoamGaash]

Nice!

[NoamGaash]

that's a very nice change
welcome!

[NoamGaash]

@all-contributors please add @jonsuguiyama as code contributor :)

[allcontributors]

@NoamGaash

I've put up a pull request to add @jonsuguiyama! 🎉