chore(web): add Vitest; add dayCapabilities helper with full test coverage

- Install vitest + jsdom
- Add test/test:watch scripts to package.json
- Add test:web and test:all tasks to mise.toml
- Add dayCapabilities() to utils.ts — single source of truth for
  what actions are permitted per day (future/today/past, open/closed)
- Add DayCapabilities interface to utils.ts
- 11 unit tests: dayCapabilities (5 cases), weekDayKeys (3 cases),
  isWorkday (3 cases)

web/src/lib/utils.test.ts

@@ -0,0 +1,84 @@
+import { describe, it, expect } from 'vitest';
+import { dayCapabilities, weekDayKeys, isWorkday } from './utils';
+
+describe('dayCapabilities', () => {
+	const today = '2026-04-30';
+
+	it('future day: only canMarkKind', () => {
+		const cap = dayCapabilities('2026-05-01', today, false);
+		expect(cap.canStartStop).toBe(false);
+		expect(cap.canAddInterval).toBe(false);
+		expect(cap.canEditEntries).toBe(false);
+		expect(cap.canMarkKind).toBe(true);
+		expect(cap.canCloseDay).toBe(false);
+		expect(cap.canReopenDay).toBe(false);
+	});
+
+	it('today, open: full access except reopen', () => {
+		const cap = dayCapabilities(today, today, false);
+		expect(cap.canStartStop).toBe(true);
+		expect(cap.canAddInterval).toBe(true);
+		expect(cap.canEditEntries).toBe(true);
+		expect(cap.canMarkKind).toBe(true);
+		expect(cap.canCloseDay).toBe(true);
+		expect(cap.canReopenDay).toBe(false);
+	});
+
+	it('today, closed: only markKind and reopen', () => {
+		const cap = dayCapabilities(today, today, true);
+		expect(cap.canStartStop).toBe(false);
+		expect(cap.canAddInterval).toBe(false);
+		expect(cap.canEditEntries).toBe(false);
+		expect(cap.canMarkKind).toBe(true);
+		expect(cap.canCloseDay).toBe(false);
+		expect(cap.canReopenDay).toBe(true);
+	});
+
+	it('past, open: no start/stop; can add/edit/mark/close', () => {
+		const cap = dayCapabilities('2026-04-29', today, false);
+		expect(cap.canStartStop).toBe(false);
+		expect(cap.canAddInterval).toBe(true);
+		expect(cap.canEditEntries).toBe(true);
+		expect(cap.canMarkKind).toBe(true);
+		expect(cap.canCloseDay).toBe(true);
+		expect(cap.canReopenDay).toBe(false);
+	});
+
+	it('past, closed: only markKind and reopen', () => {
+		const cap = dayCapabilities('2026-04-28', today, true);
+		expect(cap.canStartStop).toBe(false);
+		expect(cap.canAddInterval).toBe(false);
+		expect(cap.canEditEntries).toBe(false);
+		expect(cap.canMarkKind).toBe(true);
+		expect(cap.canCloseDay).toBe(false);
+		expect(cap.canReopenDay).toBe(true);
+	});
+});
+
+describe('weekDayKeys', () => {
+	it('2026-W18: Mon Apr 27 – Sun May 3', () => {
+		const keys = weekDayKeys('2026-W18');
+		expect(keys[0]).toBe('2026-04-27');
+		expect(keys[6]).toBe('2026-05-03');
+		expect(keys).toHaveLength(7);
+	});
+
+	it('2024-W03: Mon Jan 15 – Sun Jan 21', () => {
+		const keys = weekDayKeys('2024-W03');
+		expect(keys[0]).toBe('2024-01-15');
+		expect(keys[6]).toBe('2024-01-21');
+	});
+
+	it('2023-W01: Mon Jan 2 – Sun Jan 8', () => {
+		const keys = weekDayKeys('2023-W01');
+		expect(keys[0]).toBe('2023-01-02');
+		expect(keys[6]).toBe('2023-01-08');
+	});
+});
+
+describe('isWorkday', () => {
+	const monToFri = 31; // bits 1+2+4+8+16
+	it('Monday is a workday', () => expect(isWorkday('2026-04-27', monToFri)).toBe(true));
+	it('Saturday is not a workday', () => expect(isWorkday('2026-05-02', monToFri)).toBe(false));
+	it('Sunday is not a workday', () => expect(isWorkday('2026-05-03', monToFri)).toBe(false));
+});

web/src/lib/utils.ts

@@ -100,3 +100,61 @@ export function isWorkday(dayKey: string, mask: number): boolean {
 	const d = new Date(dayKey + 'T12:00:00Z');
 	return (weekdayBit(d.getUTCDay()) & mask) !== 0;
 }
+
+/** Capabilities for a given day in the day detail panel. */
+export interface DayCapabilities {
+	canStartStop: boolean;
+	canAddInterval: boolean;
+	canEditEntries: boolean;
+	canMarkKind: boolean;
+	canCloseDay: boolean;
+	canReopenDay: boolean;
+}
+
+/**
+ * Compute what actions are permitted for a given day.
+ *
+ * @param dayKey      The day in question (YYYY-MM-DD).
+ * @param today       Today's day key (YYYY-MM-DD).
+ * @param isClosed    Whether the day has a closed_days row.
+ */
+export function dayCapabilities(
+	dayKey: string,
+	today: string,
+	isClosed: boolean
+): DayCapabilities {
+	const isFuture = dayKey > today;
+	const isToday = dayKey === today;
+
+	if (isFuture) {
+		return {
+			canStartStop: false,
+			canAddInterval: false,
+			canEditEntries: false,
+			canMarkKind: true,
+			canCloseDay: false,
+			canReopenDay: false
+		};
+	}
+
+	if (isClosed) {
+		return {
+			canStartStop: false,
+			canAddInterval: false,
+			canEditEntries: false,
+			canMarkKind: true,
+			canCloseDay: false,
+			canReopenDay: true
+		};
+	}
+
+	// Past or today, open
+	return {
+		canStartStop: isToday,
+		canAddInterval: true,
+		canEditEntries: true,
+		canMarkKind: true,
+		canCloseDay: true,
+		canReopenDay: false
+	};
+}