fix(#172): guard mounted before snackbar in channel Block sender

Gemini review (HIGH): add !mounted guard after the async block/blockName before
showing the snackbar. Captured messenger already made it safe, but this hardens
against showing on a popped route. (MEDIUM 'stale itemBuilder read' assessed as a
false positive: context.read in itemBuilder returns live state at menu-open.)

Epic A #165 / A5 #172.

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

lib/screens/channel_chat_screen.dart

@@ -519,9 +519,11 @@ class _ChannelChatScreenState extends State<ChannelChatScreen> {
       for (final key in keys) {
         await blockService.block(key);
       }
+      if (!mounted) return;
       messenger.showSnackBar(SnackBar(content: Text(l10n.block_blocked(name))));
     } else {
       await blockService.blockName(name);
+      if (!mounted) return;
       messenger.showSnackBar(
         SnackBar(content: Text(l10n.block_blockedNameOnly(name))),
       );