feat(data): dedupe history by (date, employeeId)

tests/data.test.mjs

@@ -0,0 +1,47 @@
+import assert from 'node:assert/strict';
+import { addHistoryEntries, removeHistoryEntry } from '../js/data.js';
+
+function makeState(history = []) {
+  return {
+    employees: [],
+    calendar: { holidays: [], companyClosures: [] },
+    history: [...history]
+  };
+}
+
+// addHistoryEntries: two entries with same date but different employees both kept
+{
+  const state = makeState();
+  addHistoryEntries(state, [
+    { date: '2026-04-01', employeeId: 'alice', manual: false },
+    { date: '2026-04-01', employeeId: 'bob', manual: false }
+  ]);
+  assert.equal(state.history.length, 2, 'two distinct entries kept on same date');
+  const ids = state.history.map(h => h.employeeId).sort();
+  assert.deepEqual(ids, ['alice', 'bob'], 'both employees present');
+}
+
+// addHistoryEntries: same (date, employeeId) replaced, others on same date untouched
+{
+  const state = makeState([
+    { date: '2026-04-01', employeeId: 'alice', manual: false },
+    { date: '2026-04-01', employeeId: 'bob', manual: false }
+  ]);
+  addHistoryEntries(state, [
+    { date: '2026-04-01', employeeId: 'alice', manual: true }
+  ]);
+  assert.equal(state.history.length, 2, 'still two entries');
+  const alice = state.history.find(h => h.employeeId === 'alice');
+  assert.equal(alice.manual, true, 'alice replaced with manual=true');
+  const bob = state.history.find(h => h.employeeId === 'bob');
+  assert.equal(bob.manual, false, 'bob untouched');
+}
+
+// addHistoryEntries: empty input → no change
+{
+  const state = makeState([{ date: '2026-04-01', employeeId: 'alice', manual: false }]);
+  addHistoryEntries(state, []);
+  assert.equal(state.history.length, 1, 'empty entries → no change');
+}
+
+console.log('All data tests passed.');