feat(ui,window): add accurate unread indicators and history paging support

- Track unread message indicators and paging state in the window component
- Clear and restore unread markers correctly when navigating between conversations
- Suppress buffer updates and message printing while viewing history to prevent
  unwanted scroll jumps and viewport lock

Also includes minor code-style, whitespace, and comment cleanups as well as
updated documentation for metrics and buffer handling.

src/ui/titlebar.c

@@ -268,7 +268,11 @@ _show_scrolled(ProfWin* current)
         wattroff(win, bracket_attrs);
 
         wattron(win, scrolled_attrs);
-        wprintw(win, "SCROLLED");
+        if (current->layout->unread_msg == 0) {
+            wprintw(win, "SCROLLED");
+        } else {
+            wprintw(win, "SCROLLED, NEW MESSAGES");
+        }
         wattroff(win, scrolled_attrs);
 
         wattron(win, bracket_attrs);