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>

server/db.js

@@ -62,6 +62,7 @@ function getDefaultSettings() {
     rememberVolume: true,
     lastVolume: 80,
     autoPlayNextEpisode: false,
+    liveChannelListMode: 'grouped',
     forceProxy: false,
     forceTranscode: false, // Force Audio Transcode
     forceVideoTranscode: false, // Force Video Transcode