feat(calendar): add Nivo data adapter and verify durable API behavior

src/shared/calendar.test.ts

@@ -0,0 +1,23 @@
+import { expect, test } from 'bun:test';
+import { fixture } from '../habits/test-fixture';
+import { toResponsiveCalendarProps } from './calendar';
+test('Nivo adapter preserves per-date colors, neutral/future states and readable exact counts', async () => {
+  const f = fixture();
+  try {
+    const h = await f.json('/habits', 'POST', { name: 'Water', method: 'count', target: 8 }, 201);
+    await f.json(`/habits/${h.id}/days/2026-09-04/progress`, 'PUT', { count: 7 });
+    f.setTime('2026-09-05T12:00Z'); await f.json(`/habits/${h.id}`, 'PATCH', { target: 10 });
+    await f.json(`/habits/${h.id}/days/2026-09-05/progress`, 'PUT', { count: 7 });
+    const payload = await f.json(`/habits/${h.id}/calendar?from=2026-09-03&to=2026-09-06`);
+    const props = toResponsiveCalendarProps(payload);
+    expect(props.data).toHaveLength(4);
+    expect(props.colorScale(1)).not.toBe(props.colorScale(2));
+    expect(props.colorScale(0)).toBe(payload.settings.notDueColor); expect(props.colorScale(3)).toBe(payload.settings.futureColor);
+    expect(props.valueFormat(0)).toBe('Nothing scheduled'); expect(props.valueFormat(1)).toBe('7 of 8 steps');
+    expect(props.valueFormat(2)).toBe('7 of 10 steps'); expect(props.valueFormat(3)).toBe('Upcoming');
+    expect(props.valueFormat(99)).toBe(''); expect(props.colorScale(99)).toBe(payload.settings.notDueColor); expect(props.colorScale.ticks()).toEqual([]);
+    const c = await f.json('/charts', 'POST', { name: 'One', habitIds: [h.id] }, 201);
+    const combined = await f.json(`/charts/${c.id}/calendar?from=2026-09-04&to=2026-09-04`);
+    expect(toResponsiveCalendarProps(combined).valueFormat(0)).toBe('0 of 1 habits complete');
+  } finally { f.close(); }
+});

src/shared/calendar.ts

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+import { ResponsiveCalendar } from '@nivo/calendar';
+import type { ComponentProps } from 'react';
+import type { HabitService } from '../habits/service';
+
+export type CalendarResponse = ReturnType<HabitService['calendar']>;
+export type ResponsiveCalendarProps = ComponentProps<typeof ResponsiveCalendar>;
+
+/** Data adapter only: no frontend component or page is rendered here. */
+export function toResponsiveCalendarProps(calendar: CalendarResponse) {
+  // Nivo calls colorScale with a value, not a date. Give each date a stable
+  // index so identical shade levels under different historical targets cannot
+  // collapse into one color. Actual progress remains in calendar.days.
+  const data = calendar.days.map((day, index) => ({ day: day.date, value: index }));
+  const colorScale = Object.assign(
+    (value: number | { valueOf(): number }) => calendar.days[Number(value)]?.color ?? calendar.settings.notDueColor,
+    { ticks: () => [] as number[] },
+  );
+  const props = {
+    from: calendar.from, to: calendar.to, data, colorScale,
+    emptyColor: calendar.settings.notDueColor,
+    valueFormat: (index: number) => {
+      const day = calendar.days[index];
+      if (!day) return '';
+      if (day.future) return 'Upcoming';
+      if (day.ratio === null) return 'Nothing scheduled';
+      // Counts for combined charts are always equal-weight completed habits.
+      if (calendar.kind === 'combined') return `${day.completed} of ${day.due} habits complete`;
+      const habit = day.habits[0]!;
+      return `${habit.value} of ${habit.target} ${habit.unit}`;
+    },
+  } satisfies ResponsiveCalendarProps;
+  return props;
+}