Related Issue

Resolve #189

Summary

• Adds horizontal layout for 2D grouped bar charts (categories on Y axis, values on X axis, bars grow right).
• Exposes the feature through CLI --horizontal, --chart bar:horizontal, the UI settings panel, and ?bar.h=true deep links.
• Horizontal mode is layout-only — it flips ECharts axis geometry without remapping data or labels (distinct from --swap).
• Keeps horizontal bars unavailable for 3D bar charts, mixed mode, value mode, and non-bar chart types.

Changes

• Added bar-only --horizontal CLI flag and horizontal config field in Go (HorizontalFlag, Bar.Horizontal).
• Added --chart bar:horizontal support via the chart flag registry.
• Added Horizontal bars UI toggle for 2D bar charts; hidden when rendering 3D or in mixed/value transform modes.
• Added createHorizontalAxisConfig, createHorizontalDataZoomConfig, and right-positioned bar labels for horizontal orientation.
• Branched useBarChartOptions for horizontal grouped mode: legend at bottom center, category Y-axis (inverse: true), value X-axis, Y-axis dataZoom slider.
• Added bar.h URL deep-link in useUrlRouter.
• Left 3D bar rendering (useBar3DChartOptions) unchanged.
• Added backend and UI tests for flag registration, config materialisation, field visibility, store updates, axis/legend options, and URL routing.
• Updated bar chart docs (charts/bar.mdx, commands/charts.mdx, commands/root.mdx).
• Regenerated the embedded UI bundle in pkg/template/vizb-ui.gen.go.

Test Result

Testing Steps

• Ran full Go test suite:

go test ./...

• Ran targeted UI tests:

cd ui
corepack pnpm test \
  src/composables/charts/useBarChartOptions.test.ts \
  src/composables/useUrlRouter.test.ts \
  src/composables/settings/fieldRegistry.test.ts \
  src/components/SettingsPanel.test.ts

• Smoke-tested --horizontal CLI output:

go run . bar examples/csv/sales.csv -g region,category -p x,y --horizontal --show-labels -o out.html

• Verified generated HTML contains baked config:

grep -o '"horizontal":true' out.html

• Smoke-tested root --chart override:

go run . examples/csv/sales.csv -g region,category -p x,y -c bar --chart bar:horizontal -o dashboard.html

• Verified CLI flag exposure:

go run . bar --help | grep horizontal
go run . line --help | grep horizontal

Expected: --horizontal appears under bar --help only.