[FIX] chat: fix message pagination logic

`loadInitial()` fetched 100 messages while `_page_size` was 20,
so `_has_more` turned false too early and history stopped around
10h back. `loadMore()` also used `_currentPage * _page_size`
for offset, causing duplicates.

Use `_page_size` for initial load and rely on current message
count as offset. This removes `_currentPage` entirely and lets
older messages load correctly.

Signed-off-by: Texas0295 <kimura@texas0295.top>

lib/screens/chat/room.dart

@@ -289,7 +289,6 @@ class MessagesNotifier extends _$MessagesNotifier {
   bool? _withAttachments;
 
   late final String _roomId;
-  int _currentPage = 0;
   static const int _pageSize = 20;
   bool _hasMore = true;
   bool _isSyncing = false;
@@ -570,9 +569,11 @@ class MessagesNotifier extends _$MessagesNotifier {
   if (_searchQuery == null || _searchQuery!.isEmpty) {
     syncMessages();
   }
-    final messages = await _getCachedMessages(offset: 0, take: 100);
-    _currentPage = 0;
+
+  final messages = await _getCachedMessages(offset: 0, take: _pageSize);
+
   _hasMore = messages.length == _pageSize;
+
   state = AsyncValue.data(messages);
 }
 
@@ -582,11 +583,9 @@ class MessagesNotifier extends _$MessagesNotifier {
 
   try {
     final currentMessages = state.value ?? [];
-      _currentPage++;
-      final newMessages = await listMessages(
-        offset: _currentPage * _pageSize,
-        take: _pageSize,
-      );
+    final offset = currentMessages.length;
+
+    final newMessages = await listMessages(offset: offset, take: _pageSize);
 
     if (newMessages.isEmpty || newMessages.length < _pageSize) {
       _hasMore = false;
@@ -603,7 +602,6 @@ class MessagesNotifier extends _$MessagesNotifier {
       stackTrace: stackTrace,
     );
     showErrorAlert(err);
-      _currentPage--;
   }
 }