feat: store the hide-clutter list as a delta, not a snapshot

Saving the editor used to persist the full effective list, which froze the
shipped defaults at that moment: every default added in a later version
never reached anyone who had touched the list once.

Storage now keeps only { removedDefaults, addedDomains }, and the effective
list is resolved against whatever ships in the running version. Entries are
tagged default or user, the editor groups them under comment headers, and
'#' lines are ignored on save. Lists saved by earlier versions are migrated
to the delta form on first read and the legacy key is removed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

extension/background.js

@@ -465,7 +465,7 @@ function emitEpisodeLobbyForCurrentPrivacy() {
 const LEGACY_SYNC_KEYS = [
     'serverUrl', 'useCustomServer', 'roomId', 'password', 'chatKey',
     'chatEnabled', 'chatNotifications', 'chatPosition', 'chatSize', 'chatStartMode', 'chatReactionDisplay', 'username',
-    'filterNoise', 'customBlacklistDomains', 'autoSyncNextEpisode', 'forceSyncMode',
+    'filterNoise', 'customBlacklistDomains', 'blacklistOverrides', 'autoSyncNextEpisode', 'forceSyncMode',
     'browserNotifications', 'autoCopyInvite', 'locale', 'audioSettings',
     'titlePrivacyMode', 'sendTabTitle', 'mediaTitlePrivacyMode'
 ];