feat: add settings option to hide Live TV categories (#11)

Add a liveChannelListMode setting (default 'grouped') with a Settings
toggle to display the Live TV channel list flat, without category
group sections. When enabled, logical channels are listed directly
with their stream variants still grouped as in #9/#10. Cover the
setting default and flat-list behavior with tests.

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

test/settingsDefaults.test.js

@@ -0,0 +1,7 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const { getDefaultSettings } = require('../server/db');
+
+test('defaults Live TV channel lists to grouped mode', () => {
+    assert.equal(getDefaultSettings().liveChannelListMode, 'grouped');
+});