Build fl_chart Adapter with Design Tokens

fl_chart's BarChartData is immutable and constructed via named parameters — build it entirely inside the factory method without storing intermediate mutable state. Access design tokens via `Theme.of(context).extension()` or the equivalent pattern used elsewhere in the app — do not use a global singleton. For multi-org theme support, the adapter must accept a `BuildContext` (to resolve the current theme) rather than accepting raw Color parameters — this keeps theme switching automatic. For the donut chart centre hole, use `PieChartData(sectionsSpace: 2, centerSpaceRadius: 40)` where `40` should reference a token like `AppSizing.chartCentreRadius`.

For WCAG validation, verify each organisation's accent colour passes contrast against `AppColors.chartBackground` before committing — use a helper: `contrastRatio(Color fg, Color bg)` based on WCAG 2.1 relative luminance formula. Avoid defining chart colours inside FlChartAdapter — delegate colour selection to the calling ViewModel layer so the adapter remains a pure formatting utility.

Widget tests with flutter_test: (1) Render buildBarChartData output inside BarChart widget with a known list of BarChartSeriesViewModel — assert widget builds without error and the correct number of bar groups is present. (2) Render buildDonutChartData output inside PieChart — assert the correct number of PieSections with non-zero percentages. (3) Pass empty BarChartSeriesViewModel list — assert no exception and a valid empty chart renders. (4) For each of the three organisation themes (NHF, HLF, Blindeforbundet), override the DesignTokenTheme and assert that colours in the returned chart data change accordingly.

(5) Assert WCAG contrast ratio for the primary bar fill colour against the chart background using a helper function that computes relative luminance. Golden file tests are optional but recommended for tooltip renderer output.