feat: persist active main tab to localStorage

On tab click, store the tab name under 'activeMainTab' in localStorage.
On DOMContentLoaded, restore the previously active tab instead of
always defaulting to 'queue'.

Exported getActiveMainTab/setActiveMainTab for testability, following
the same pattern as getTaskFilterTab/setTaskFilterTab.

Tests: web/test/tab-persistence.test.mjs (6 tests, all green).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 58 insertions, 0 deletions

diff --git a/web/test/tab-persistence.test.mjs b/web/test/tab-persistence.test.mjs
new file mode 100644
index 0000000..9311453
--- /dev/null
+++ b/web/test/tab-persistence.test.mjs
@@ -0,0 +1,58 @@
+// tab-persistence.test.mjs — TDD tests for main-tab localStorage persistence
+//
+// Run with: node --test web/test/tab-persistence.test.mjs
+
+import { describe, it, beforeEach } from 'node:test';
+import assert from 'node:assert/strict';
+
+// ── localStorage mock ──────────────────────────────────────────────────────────
+// Must be set up before importing app.js so the module sees the global.
+const store = new Map();
+globalThis.localStorage = {
+  getItem:    (k)    => store.has(k) ? store.get(k) : null,
+  setItem:    (k, v) => store.set(k, String(v)),
+  removeItem: (k)    => store.delete(k),
+  clear:      ()     => store.clear(),
+};
+
+import { getActiveMainTab, setActiveMainTab } from '../app.js';
+
+describe('getActiveMainTab', () => {
+  beforeEach(() => store.clear());
+
+  it('returns "queue" when localStorage has no stored value', () => {
+    assert.equal(getActiveMainTab(), 'queue');
+  });
+
+  it('returns the tab name stored by setActiveMainTab', () => {
+    setActiveMainTab('settings');
+    assert.equal(getActiveMainTab(), 'settings');
+  });
+
+  it('returns "queue" after localStorage value is removed', () => {
+    setActiveMainTab('stats');
+    localStorage.removeItem('activeMainTab');
+    assert.equal(getActiveMainTab(), 'queue');
+  });
+
+  it('reflects the most recent setActiveMainTab call', () => {
+    setActiveMainTab('stats');
+    setActiveMainTab('running');
+    assert.equal(getActiveMainTab(), 'running');
+  });
+});
+
+describe('setActiveMainTab', () => {
+  beforeEach(() => store.clear());
+
+  it('writes the tab name to localStorage under key "activeMainTab"', () => {
+    setActiveMainTab('drops');
+    assert.equal(localStorage.getItem('activeMainTab'), 'drops');
+  });
+
+  it('overwrites a previously stored tab', () => {
+    setActiveMainTab('queue');
+    setActiveMainTab('interrupted');
+    assert.equal(localStorage.getItem('activeMainTab'), 'interrupted');
+  });
+});