chatgui: remember last read message, restore scroll position, show unread count badge

Store read/scroll positions per channel via ui_state table (key-value).
Draw red +N badge on channel list items for unread messages.
Fix getMessages() to fetch last N messages (was fetching first N).

tools/chatgui/src/channeldelegate.cpp

@@ -125,6 +125,28 @@ void ChannelDelegate::paint(QPainter *painter, const QStyleOptionViewItem &optio
         }
     }
 
+    /* ── unread count badge (right of channel name) ── */
+    int unread = index.data(ChannelUnreadCountRole).toInt();
+    if (unread > 0) {
+        QString unreadText = unread > 99 ? QStringLiteral("99+") : QString::number(unread);
+        QFont badgeFont = option.font;
+        badgeFont.setBold(true);
+        badgeFont.setPixelSize(11);
+        QFontMetrics badgeFm(badgeFont);
+        int textW = badgeFm.horizontalAdvance(unreadText) + 8;
+        int badgeH = 18;
+        if (textW < badgeH) textW = badgeH;
+        QRect badgeRect(textRect.right() - textW, nameRect.top(),
+                        textW, badgeH);
+        painter->setRenderHint(QPainter::Antialiasing);
+        painter->setBrush(QColor("#E53935"));
+        painter->setPen(Qt::NoPen);
+        painter->drawRoundedRect(badgeRect, badgeH / 2, badgeH / 2);
+        painter->setFont(badgeFont);
+        painter->setPen(Qt::white);
+        painter->drawText(badgeRect, Qt::AlignCenter, unreadText);
+    }
+
     /* ── invite plus button (bottom-right) ── */
     QRect plusRect = inviteButtonRect(option.rect);
     bool plusHovered = index.data(ChannelInviteBtnHoveredRole).toInt() != 0;