test: add jsdom render + persistence integration tests (#13)

- render.test.mjs drives the real UI under jsdom: employees tab renders and
  add works, planning tab renders the calendar grid (makeDayCell), generate
  fills slots, a manual slot change updates history, and a beforeunload flush
  persists state to localStorage; data tab renders the history view.
- persistence.test.mjs seeds malformed persisted state (employee without
  constraints, out-of-range moderatorsPerDay) and asserts the startup load
  normalizes it without crashing.
- jsdom added as the single devDependency; node_modules gitignored; both
  suites wired into npm test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/persistence.test.mjs

@@ -0,0 +1,43 @@
+import assert from 'node:assert/strict';
+import { JSDOM } from 'jsdom';
+
+const HTML = `<!DOCTYPE html><html><body>
+  <button class="tab-btn active" data-tab="employees">Mitarbeiter</button>
+  <button class="tab-btn" data-tab="planning">Monatsplanung</button>
+  <button class="tab-btn" data-tab="data">Daten</button>
+  <section id="tab-employees" class="tab-content"></section>
+  <section id="tab-planning" class="tab-content hidden"></section>
+  <section id="tab-data" class="tab-content hidden"></section>
+</body></html>`;
+
+const dom = new JSDOM(HTML, { url: 'http://localhost/' });
+global.window = dom.window;
+global.document = dom.window.document;
+global.localStorage = dom.window.localStorage;
+dom.window.alert = () => {};
+global.alert = () => {};
+
+// Seed malformed persisted state BEFORE app.js loads:
+// - an employee with no constraints (would crash plan generation if not normalized)
+// - an out-of-range moderatorsPerDay
+localStorage.setItem('morning-planner-state', JSON.stringify({
+  employees: [{ id: 'x', name: 'NoConstraints' }],
+  calendar: { holidays: [], companyClosures: [] },
+  history: [],
+  settings: { moderatorsPerDay: 99 }
+}));
+
+const { state } = await import('../js/app.js');
+
+// Employee is loaded but normalized (constraints filled) so nothing crashes later
+assert.equal(state.employees.length, 1, 'employee loaded from storage');
+assert.ok(Array.isArray(state.employees[0].constraints.neverDays), 'missing constraints normalized');
+assert.equal(state.employees[0].constraints.minGapDays, 14, 'minGapDays defaulted');
+
+// Invalid moderatorsPerDay (99) falls back to the valid default
+assert.equal(state.settings.moderatorsPerDay, 1, 'out-of-range setting falls back to 1');
+
+// Rendering the employee tab with the normalized data does not throw
+assert.ok(document.getElementById('emp-name-input'), 'app rendered after recovering from malformed state');
+
+console.log('All persistence tests passed.');